fscache, cachefiles: Fix checker warnings

Fix a couple of checker warnings in fscache and cachefiles:

 (1) fscache_n_op_requeue is never used, so get rid of it.

 (2) cachefiles_uncache_page() is passed in a lock that it releases, so
     this needs annotating.

Signed-off-by: David Howells <dhowells@redhat.com>