null_blk: add zoned config support information

If the kernel is built without CONFIG_BLK_DEV_ZONED, a modprobe
of the null_blk driver with zoned=1 fails with 'Invalid argument'.
This can be confusing to users, prompting a search as to why the
parameter is invalid. To assist in that search, add a bit more
information to the failure, additionally adding to the documentation
that CONFIG_BLK_DEV_ZONED is needed for zoned=1.

Reviewed-by: Bart Van Assche <bvanassche@acm.org>
Signed-off-by: John Pittman <jpittman@redhat.com>

Added null_blk prefix to error message.

Signed-off-by: Jens Axboe <axboe@kernel.dk>