raid5: Add R5_ReadNoMerge flag which prevent bio from merging at block layer

Because bios will merge at block-layer,so bios-error may caused by other
bio which be merged into to the same request.
Using this flag,it will find exactly error-sector and not do redundant
operation like re-write and re-read.

V0->V1:Using REQ_FLUSH instead REQ_NOMERGE avoid bio merging at block
layer.

Signed-off-by: Jianpeng Ma <majianpeng@gmail.com>
Signed-off-by: NeilBrown <neilb@suse.de>