ceph: don't improperly set dir complete when holding EXCL cap

If we hold the EXCL cap, we cannot trust the dir stats from the MDS (num
files, subdirs) and must not incorrectly conclude that the directory is
empty.  If we do, we get can bad results from lookup (bad ENOENT) and
bad readdir results.

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