ceph: fix f_namelen reported by statfs

We were setting f_namelen in kstatfs to PATH_MAX instead of NAME_MAX.
That disagrees with ceph_lookup behavior (which checks against NAME_MAX),
and also makes the pjd posix test suite spit out ugly errors because with
can't clean up its temporary files.

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