nvmet: print a hint while rejecting NSID 0 or 0xffffffff

Adding this hint for the sake of convenience.

It was spotted that a few times people spent some time before
understanding what is exactly wrong in configuration process.  This
should save a few time in such situations, especially for people who
is not very confident with NVMe requirements.

Signed-off-by: Mikhail Skorzhinskii <mskorzhinskiy@solarflare.com>
Reviewed-by: Sagi Grimberg <sagi@grimberg.me>
Signed-off-by: Christoph Hellwig <hch@lst.de>