nvmet: make nvmet_wq unbound

When deleting many controllers one-by-one, it takes a very
long time as these work elements may serialize as they are
scheduled on the executing cpu instead of spreading. In general
nvmet_wq can definitely be used for long standing work elements
so its better to make it unbound regardless.

Signed-off-by: Sagi Grimberg <sagi.grimberg@vastdata.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Keith Busch <kbusch@kernel.org>