client: skip lookupname if writing to unlinked file

client: skip lookupname if writing to unlinked file

When a client writes to an unlinked file, get_quota_root tries to
resolve the filename via an MDS lookupname op. The op always results in
-13 permission denied for path-restricted caps or -2 no such file or
directory otherwise. More importantly, the repeated lookupname ops slow
down buffered writes significantly.

Don't do the lookupname for unlinked files; use the root_ancentor's
quota instead.

Fixes: http://tracker.ceph.com/issues/20945
Backport: jewel, luminous
Signed-off-by: Dan van der Ster <daniel.vanderster@cern.ch>