ceph: cap revocation fixes

Try to invalidate pages in ceph_check_caps() if FILE_CACHE is being
revoked.  If we fail, queue an immediate async invalidate if FILE_CACHE
is being revoked.  (If it's not being revoked, we just queue the caps
for later evaluation later, as per the old behavior.)

Signed-off-by: Sage Weil <sage@newdream.net>