docs/bpf: Fix sphinx warnings for devmap

Sphinx version >=3.1 warns about duplicate function declarations in the
DEVMAP documentation. This is because the function name is the same for
kernel and user space BPF progs but the parameters and return types
they take is what differs. This patch moves from using the ``c:function::``
directive to using the ``code-block:: c`` directive. The patches also fix
the indentation for the text associated with the "new" code block delcarations.
The missing support of c:namespace-push:: and c:namespace-pop:: directives by
helper scripts for kernel documentation prevents using the ``c:function::``
directive with proper namespacing.

Signed-off-by: Maryam Tahhan <mtahhan@redhat.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20221123092321.88558-3-mtahhan@redhat.com

Documentation/bpf/map_devmap.rst

@@ -29,7 +29,10 @@ Usage
 =====
 Kernel BPF
 ----------
-.. c:function::
+bpf_redirect_map()
+^^^^^^^^^^^^^^^^^^
+.. code-block:: c
+
     long bpf_redirect_map(struct bpf_map *map, u32 key, u64 flags)
 
 Redirect the packet to the endpoint referenced by ``map`` at index ``key``.
@@ -56,20 +59,26 @@ lower bits of the ``flags`` argument if the map lookup fails.
 
 More information about redirection can be found :doc:`redirect`
 
-.. c:function::
+bpf_map_lookup_elem()
+^^^^^^^^^^^^^^^^^^^^^
+.. code-block:: c
+
    void *bpf_map_lookup_elem(struct bpf_map *map, const void *key)
 
 Net device entries can be retrieved using the ``bpf_map_lookup_elem()``
 helper.
 
 User space
----------
+----------
 .. note::
     DEVMAP entries can only be updated/deleted from user space and not
     from an eBPF program. Trying to call these functions from a kernel eBPF
     program will result in the program failing to load and a verifier warning.
 
-.. c:function::
+bpf_map_update_elem()
+^^^^^^^^^^^^^^^^^^^^^
+.. code-block:: c
+
    int bpf_map_update_elem(int fd, const void *key, const void *value, __u64 flags);
 
 Net device entries can be added or updated using the ``bpf_map_update_elem()``
@@ -88,7 +97,6 @@ Userspace
     };
 
 The ``flags`` argument can be one of the following:
-
   - ``BPF_ANY``: Create a new element or update an existing element.
   - ``BPF_NOEXIST``: Create a new element only if it did not exist.
   - ``BPF_EXIST``: Update an existing element.
@@ -104,12 +112,20 @@ Userspace
 - ``tools/testing/selftests/bpf/prog_tests/xdp_devmap_attach.c``
 - ``tools/testing/selftests/bpf/progs/test_xdp_with_devmap_helpers.c``
 
+bpf_map_lookup_elem()
+^^^^^^^^^^^^^^^^^^^^^
+.. code-block:: c
+
 .. c:function::
    int bpf_map_lookup_elem(int fd, const void *key, void *value);
 
 Net device entries can be retrieved using the ``bpf_map_lookup_elem()``
 helper.
 
+bpf_map_delete_elem()
+^^^^^^^^^^^^^^^^^^^^^
+.. code-block:: c
+
 .. c:function::
    int bpf_map_delete_elem(int fd, const void *key);