libnvme: Rework qpairs to lock themselves.

They were theoretically guarded by the controller lock, but it appears
nvme_ns bypassed that, meaning that if ns_read was executed at the
same time as qpair_poll, unpredictable races could occur. This solves
that by making the qpairs guarded by their own mutex, which also
has the advantage of poll() being executable on more than one qpair
at a time.

Seems to fix the KDLs in #15123 (and maybe other NVMe tickets),
though the I/O corruptions remain.

PVS V519: setting the same variable twice

Change-Id: I3967f1c77c89d1aa9d040d8ff7a8593daca8f8b2
Reviewed-on: https://review.haiku-os.org/c/1603
Reviewed-by: Stephan Aßmus <superstippi@gmx.de>

libnvme: make configure_aer failing a warning.

The spec generally requires this feature, but some emulated hardware
(e.g. QEMU) does not support it. SPDK seems to just ignore the
error and continue on with a warning, so let's do the same here.

libnvme: Import.

Minor modifications for Haiku included, and for the most part marked
as such.