vfs: fix check_acl compile error when CONFIG_FS_POSIX_ACL is not set

Commit e77819e5 ("vfs: move ACL cache lookup into generic code")
didn't take the FS_POSIX_ACL config variable into account - when that is
not set, ACL's go away, and the cache helper functions do not exist,
causing compile errors like

  fs/namei.c: In function 'check_acl':
  fs/namei.c:191:10: error: implicit declaration of function 'negative_cached_acl'
  fs/namei.c:196:2: error: implicit declaration of function 'get_cached_acl'
  fs/namei.c:196:6: warning: assignment makes pointer from integer without a cast
  fs/namei.c:212:11: error: implicit declaration of function 'set_cached_acl'

Reported-by: Markus Trippelsdorf <markus@trippelsdorf.de>
Acked-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>