nvme-core: optionally poll sync commands

Pass poll bool to indicate that we need it to poll. This prepares us for
polling support in nvmf since connect is an I/O that will be queued
and has to be polled in order to complete. If poll is passed,
we call nvme_execute_rq_polled which sends the requests and polls
for its completion.

Signed-off-by: Sagi Grimberg <sagi@grimberg.me>
Signed-off-by: Christoph Hellwig <hch@lst.de>