um/mm: remove redundant assignment of max_low_pfn

Current calculation of max_low_pfn is introduced in commit af84eab208
("[PATCH] uml: fix LVM crash"). It is intended to set max_low_pfn to the
same value as max_pfn.

But I am not sure why the max_pfn is set to totalram_pages, which
represents the number of usable pages in system instead of an absolute
page frame number. (The change history stops there.)

While we have already calculate it in setup_physmem(), so not necessary
to do it again.

Also this would help changing totalram_pages accounting, since we plan
to move the accounting into __free_pages_core(). With this change,
totalram_pages may not represent the total usable pages at this point,
since some pages would be deferred initialized.

Signed-off-by: Wei Yang <richard.weiyang@gmail.com>
CC: Jeff Dike <jdike@linux.intel.com>
Cc: Paolo 'Blaisorblade' Giarrusso <blaisorblade@yahoo.it>
Cc: Alasdair G Kergon <agk@redhat.com>
CC: Andrew Morton <akpm@linux-foundation.org>
CC: Mike Rapoport (IBM) <rppt@kernel.org>
CC: David Hildenbrand <david@redhat.com>
Acked-by: Mike Rapoport (IBM) <rppt@kernel.org>
Link: https://patch.msgid.link/20240615034150.2958-1-richard.weiyang@gmail.com
Signed-off-by: Johannes Berg <johannes.berg@intel.com>

arch/um/kernel/mem.c

@@ -73,7 +73,6 @@ void __init mem_init(void)
 
 	/* this will put all low memory onto the freelists */
 	memblock_free_all();
-	max_low_pfn = totalram_pages();
 	max_pfn = max_low_pfn;
 	kmalloc_ok = 1;
 }