[PATVH V5] nvme-core: use xarray for ctrl ns tracking

[Sagi Grimberg]

> This patch replaces the ctrl->namespaces tracking from linked list to
> xarray for better ns-mgmt on the host side. For host side
> nvme_find_get_ns() falls into the fast path for NVMeOF passthru target.
> This allows us to have better performance for NVMeOF passthru backend
> since XArray has shows better performance numbers over having
> a combination of read-write semapore read + linked list in the
> nvme_find_get_ns() to find namespace in I/O patch from nsid specified
> in the nvme_rw_cmd.

This patch scares me to be honest...

We have such non-trivial synchronization paths for ns scanning, ns
removal, quiescing, freezing, path handling... I'm not even sure
how many tests we'll need to verify that nothing breaks here.