rbtree: inline rb_last()

This is a very small function, inlining it saves cpu cycles in TCP by
reducing register pressure and removing call/ret overhead.

It also reduces vmlinux text size by 122 bytes on a typical x86_64 build.

Before:

size vmlinux
   text    data     bss     dec     hex filename
34811781        22177365        5685248 62674394        3bc55da vmlinux

After:

size vmlinux
   text	   data	    bss	    dec	    hex	filename
34811659	22177365	5685248	62674272	3bc5560	vmlinux

[ojeda@kernel.org: fix rust build]
  Link: https://lkml.kernel.org/r/20251120085518.1463498-1-ojeda@kernel.org
Link: https://lkml.kernel.org/r/20251114140646.3817319-3-edumazet@google.com
Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>
Reviewed-by: Kuan-Wei Chiu <visitorckw@gmail.com>
Cc: Jakub Kacinski <kuba@kernel.org>
Cc: Neal Cardwell <ncardwell@google.com>
Cc: Paolo Abeni <pabeni@redhat.com>
Cc: Alice Ryhl <aliceryhl@google.com>
Cc: Stehen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

rust/helpers/rbtree.c

@@ -12,3 +12,8 @@ struct rb_node *rust_helper_rb_first(const struct rb_root *root)
 {
 	return rb_first(root);
 }
+
+struct rb_node *rust_helper_rb_last(const struct rb_root *root)
+{
+	return rb_last(root);
+}