nvme-fcloop: fix "inconsistent {IN-HARDIRQ-W} -> {HARDIRQ-ON-W} usage"

fcloop_fcp_op() could be called from flush request's ->end_io(flush_end_io) in
which the spinlock of fq->mq_flush_lock is grabbed with irq saved/disabled.

So fcloop_fcp_op() can't call spin_unlock_irq(&tfcp_req->reqlock) simply
which enables irq unconditionally.

Fixes the warning by switching to spin_lock_irqsave()/spin_unlock_irqrestore()

Fixes: c38dbbfa ("nvme-fcloop: fix inconsistent lock state warnings")
Reported-by: Yi Zhang <yi.zhang@redhat.com>
Signed-off-by: Ming Lei <ming.lei@redhat.com>
Reviewed-by: Ewan D. Milne <emilne@redhat.com>
Tested-by: Yi Zhang <yi.zhang@redhat.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>