Use GFP_ATOMIC for allocations that are required for swap on zvols

dmu_bio_clone(), __vdev_disk_physio() and vdev_disk_io_flush() are used
when doing swap to a zvol. Failing to use GFP_ATOMIC permits them to
block under memory pressure, which can prevent swap operations and cause
deadlocks. We use GFP_ATOMIC | __GFP_HIGH for these allocations because
they are required for swap operations on zvols.

Signed-off-by: Richard Yao <ryao@cs.stonybrook.edu>