nfsd: don't advertise a SCSI layout for an unsupported request_queue

Commit 30181faa ("nfsd: Check queue type before submitting a SCSI
request") did the work of ensuring that we don't send SCSI requests to a
request queue that won't support them, but that check is in the
GETDEVICEINFO path.  Let's not set the SCSI layout in fs_layout_type in the
first place, and then we'll have less clients sending GETDEVICEINFO for
non-SCSI request queues and less unnecessary WARN_ONs.

While we're in here, remove some outdated comments that refer to
"overwriting" layout seletion because commit 8a4c3926 ("nfsd: allow
nfsd to advertise multiple layout types") changed things to no longer
overwrite the layout type.

Signed-off-by: Benjamin Coddington <bcodding@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>