ceph osd: add support for new op writesame

ceph osd: add support for new op writesame

This adds a new ceph request writesame that writes a buffer of length
writesame.data_length bytes at writesame.offset over
writesame.length bytes.

This command maps to SCSI's WRITE SAME request, so users like LIO+rbd
can pass this to the OSD. Right now, it only saves having to transfer
writesame.length bytes over the network, but future versions will be
to fully offload it by passing it directly to the FS/devices if they
support it.

v2:
- Fix tab/spaces to matching coding style.
- Allow zero write length. Check for invalid data lengths.

Signed-off-by: Mike Christie <mchristi@redhat.com>
Reviewed-by: David Disseldorp <ddiss@suse.de>