nvme: Pass malloc flags to request allocation functions

There are some contexts where it is safe to sleep, so we should pass
M_WAITOK to ensure that a null pointer dereference can't happen.

A few places allocate with M_NOWAIT but have no way to signal an error.
Flag those with an XXX comment.

PR: 276770
Reviewed by: imp
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D47307