ceph: take reference on mds request r_unsafe_dir

We put ourselves on an inode list for the parent directory of metadata
operations so that an fsync on the directory will wait for metadata updates
to commit to disk.  We weren't holding a reference to that directory,
however, and under certain workloads (fsstress in this case) the directory
can go away.

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