block: Reject misaligned write requests with BDRV_REQ_NO_FALLBACK

block: Reject misaligned write requests with BDRV_REQ_NO_FALLBACK

The BDRV_REQ_NO_FALLBACK flag means that an operation should only be
performed if it can be offloaded or otherwise performed efficiently.

However a misaligned write request requires a RMW so we should return
an error and let the caller decide how to proceed.

This hits an assertion since commit c8bb23cbdb if the required
alignment is larger than the cluster size:

qemu-img create -f qcow2 -o cluster_size=2k img.qcow2 4G
qemu-io -c "open -o driver=qcow2,file.align=4k blkdebug::img.qcow2" \
        -c 'write 0 512'
qemu-io: block/io.c:1127: bdrv_driver_pwritev: Assertion `!(flags & BDRV_REQ_NO_FALLBACK)' failed.
Aborted

The reason is that when writing to an unallocated cluster we try to
skip the copy-on-write part and zeroize it using BDRV_REQ_NO_FALLBACK
instead, resulting in a write request that is too small (2KB cluster
size vs 4KB required alignment).

Signed-off-by: Alberto Garcia <berto@igalia.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>