rgw_file: allow setattr on placeholder directories

rgw_file: allow setattr on placeholder directories

When a POSIX path <bucket>/foo/ is known only as an implicit path
segment from other objects (e.g., <bucket>/foo/bar.txt), a case
that would usually arise from S3 upload of such an object, an
RGWFileHandle object representing "<bucket>/foo/" will be constructed
as needed, with no backing in RGW.

This is by design, but subsequently, if a setattr is performed on
such a handle, we must be ready to create the object inline with
storing the attributes.

Fixes: http://tracker.ceph.com/issues/18989
Signed-off-by: Matt Benjamin <mbenjamin@redhat.com>
(cherry picked from commit 55eec1c0a0e136736961423b7b6244d0f3693c1a)
Signed-off-by: Matt Benjamin <mbenjamin@redhat.com>