rgw_file: introduce fast S3 Unix stats (immutable)

rgw_file: introduce fast S3 Unix stats (immutable)

For objects originating in S3/Swift, it is frequently acceptable
to synthesize c/mtime and size stats from meta attributes in the RGW
bucket index entry.

Synthetic stat behavior is not compatible with mutation of file
ownership and permissions via SETATTR.  In future, current xattr-based,
mutable, Unix stats may be moved to the bucket index.  For now, use
synthetic stats only when a new option rgw_nfs_s3_fast_attrs is present.

Fixes: http://tracker.ceph.com/issues/40456
Signed-off-by: Matt Benjamin <mbenjamin@redhat.com>