fs/dcache: avoid unused-function warning

fs/dcache: avoid unused-function warning

Now that 'dentry_stat' is marked 'static', we can run into this warning:

fs/dcache.c:128:29: error: 'dentry_stat' defined but not used [-Werror=unused-variable]
  128 | static struct dentry_stat_t dentry_stat = {

Hide it in the same #ifdef as its only references.

Link: https://lkml.kernel.org/r/20211203190123.874239-2-arnd@kernel.org
Fixes: f0eea17ca8da ("fs: move dcache 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>