rgw/s3: RGWGetObj still sets attrs on ReadOp error

rgw/s3: RGWGetObj still sets attrs on ReadOp error

when `ReadOp::prepare()` fails with `-ERR_NOT_MODIFIED`, it succeeded in
reading the head object for mtime and attrs like etag. make those attrs
available to RGWGetObj so we can still send ETag and other cache-related
response headers

Fixes: https://tracker.ceph.com/issues/45736
Signed-off-by: Casey Bodley <cbodley@redhat.com>
(cherry picked from commit ccd241c565aa898231e4bfa4ea313c3406b55391)

Conflicts:
	src/rgw/rgw_op.cc no multipart get part