ceph: only release unused caps with mds requests

We were releasing used caps (e.g. FILE_CACHE) from encode_inode_release
with MDS requests (e.g. setattr).  We don't carry refs on most caps, so
this code worked most of the time, but for setattr (utimes) we try to
drop Fscr.

This causes cap state to get slightly out of sync with reality, and may
result in subsequent mds revoke messages getting ignored.

Fix by only releasing unused caps.

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