target/user: Return an error if cmd data size is too large

Userspace should be implementing VPD B0 (Block Limits) to inform the
initiator of max data size, but just in case we do get a too-large request,
do what the spec says and return INVALID_CDB_FIELD.

Make sure to unlock udev->cmdr_lock before returning.

Signed-off-by: Andy Grover <agrover@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Mike Christie <mchristi@redhat.com>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>