include/linux/mm.h: move nr_free_buffer_pages from swap.h to mm.h

nr_free_buffer_pages could be exposed through mm.h instead of swap.h.
The advantage of this change is that it can reduce the obsolete
includes.  For example, net/ipv4/tcp.c wouldn't need swap.h any more
since it has already included mm.h.  Similarly, after checking all the
other files, it comes that tcp.c, udp.c meter.c ,...  follow the same
rule, so these files can have swap.h removed too.

Moreover, after preprocessing all the files that use
nr_free_buffer_pages, it turns out that those files have already
included mm.h.Thus, we can move nr_free_buffer_pages from swap.h to mm.h
safely.  This change will not affect the compilation of other files.

Link: https://lkml.kernel.org/r/20210912133640.1624-1-liumh1@shanghaitech.edu.cn
Signed-off-by: Mianhan Liu <liumh1@shanghaitech.edu.cn>
Cc: Jakub Kicinski <kuba@kernel.org>
CC: Ulf Hansson <ulf.hansson@linaro.org>
Cc: "David S . Miller" <davem@davemloft.net>
Cc: Simon Horman <horms@verge.net.au>
Cc: Pravin B Shelar <pshelar@ovn.org>
Cc: Vlad Yasevich <vyasevich@gmail.com>
Cc: Marcelo Ricardo Leitner <marcelo.leitner@gmail.com>
Cc: Matthew Wilcox <willy@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

drivers/mmc/core/mmc_test.c

@@ -10,7 +10,6 @@
 #include <linux/slab.h>
 
 #include <linux/scatterlist.h>
-#include <linux/swap.h>		/* For nr_free_buffer_pages() */
 #include <linux/list.h>
 
 #include <linux/debugfs.h>

include/linux/mm.h

@@ -875,6 +875,8 @@ void put_pages_list(struct list_head *pages);
 void split_page(struct page *page, unsigned int order);
 void copy_huge_page(struct page *dst, struct page *src);
 
+unsigned long nr_free_buffer_pages(void);
+
 /*
  * Compound pages have a destructor function.  Provide a
  * prototype for that function and accessor functions.

include/linux/swap.h

@@ -335,7 +335,6 @@ void workingset_update_node(struct xa_node *node);
 
 /* linux/mm/page_alloc.c */
 extern unsigned long totalreserve_pages;
-extern unsigned long nr_free_buffer_pages(void);
 
 /* Definition of global_zone_page_state not available yet */
 #define nr_free_pages() global_zone_page_state(NR_FREE_PAGES)

net/ipv4/tcp.c

@@ -260,7 +260,6 @@
 #include <linux/random.h>
 #include <linux/memblock.h>
 #include <linux/highmem.h>
-#include <linux/swap.h>
 #include <linux/cache.h>
 #include <linux/err.h>
 #include <linux/time.h>

net/ipv4/udp.c

@@ -78,7 +78,6 @@
 #include <asm/ioctls.h>
 #include <linux/memblock.h>
 #include <linux/highmem.h>
-#include <linux/swap.h>
 #include <linux/types.h>
 #include <linux/fcntl.h>
 #include <linux/module.h>

net/netfilter/ipvs/ip_vs_ctl.c

@@ -24,7 +24,6 @@
 #include <linux/sysctl.h>
 #include <linux/proc_fs.h>
 #include <linux/workqueue.h>
-#include <linux/swap.h>
 #include <linux/seq_file.h>
 #include <linux/slab.h>
 

net/openvswitch/meter.c

@@ -12,7 +12,6 @@
 #include <linux/openvswitch.h>
 #include <linux/netlink.h>
 #include <linux/rculist.h>
-#include <linux/swap.h>
 
 #include <net/netlink.h>
 #include <net/genetlink.h>

net/sctp/protocol.c

@@ -33,7 +33,6 @@
 #include <linux/seq_file.h>
 #include <linux/memblock.h>
 #include <linux/highmem.h>
-#include <linux/swap.h>
 #include <linux/slab.h>
 #include <net/net_namespace.h>
 #include <net/protocol.h>