nvme-fcloop: resolve warnings on RCU usage and sleep warnings

nvme-fcloop: resolve warnings on RCU usage and sleep warnings

With additional debugging enabled, seeing warnings for suspicious RCU
usage or Sleeping function called from invalid context.

These both map to allocation of a work structure which is currently
GFP_KERNEL, meaning it can sleep. For the RCU warning, the sequence was
sleeping while holding the RCU lock.

Convert the allocation to GFP_ATOMIC.

Signed-off-by: James Smart <jsmart2021@gmail.com>
Reviewed-by: Minwoo Im <minwoo.im.dev@gmail.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>