MFC r281280:
MFC r281280:

  nvme: fall back to a smaller MSI-X vector allocation if necessary

  Previously, if per-CPU MSI-X vectors could not be allocated,
  nvme(4) would fall back to INTx with a single I/O queue pair.
  This change will still fall back to a single I/O queue pair, but
  allocate MSI-X vectors instead of reverting to INTx.

Sponsored by:	Intel

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