ceph: fix open file counting on snapped inodes when mds returns no caps

It's possible the MDS will not issue caps on a snapped inode, in which case
an open request may not __ceph_get_fmode(), botching the open file
counting.  (This is actually a server bug, but the client shouldn't BUG out
in this case.)

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