fs/inode: avoid unused-variable warning

fs/inode: avoid unused-variable warning

Now that 'inodes_stat' is marked 'static', it causes a harmless warning
whenever it is unused:

fs/inode.c:73:29: error: 'inodes_stat' defined but not used [-Werror=unused-variable]
   73 | static struct inodes_stat_t inodes_stat;

Move it into the #ifdef that guards its only references.

Link: https://lkml.kernel.org/r/20211203190123.874239-1-arnd@kernel.org
Fixes: 245314851782 ("fs: move inode sysctls to its own file")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Cc: Luis Chamberlain <mcgrof@kernel.org>
Cc: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>