block: Allow BDRV_REQ_FUA through blk_pwrite()
We have several block drivers that understand BDRV_REQ_FUA, and emulate it in the block layer for the rest by a full flush. But without a way to actually request BDRV_REQ_FUA during a pass-through blk_pwrite(), FUA-aware block drivers like NBD are forced to repeat the emulation logic of a full flush regardless of whether the backend they are writing to could do it more efficiently. This patch just wires up a flags argument; followup patches will actually make use of it in the NBD driver and in qemu-io. Signed-off-by: NEric Blake <eblake@redhat.com> Acked-by: NDenis V. Lunev <den@openvz.org> Signed-off-by: NKevin Wolf <kwolf@redhat.com>
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