md: pass down BIO_RW_SYNC in raid{1,10}

md: pass down BIO_RW_SYNC in raid{1,10}

md raidX make_request functions strip off the BIO_RW_SYNC flag, thus
introducing additional latency.

Fixing this in raid1 and raid10 seems to be straightforward enough.

For our particular usage case in DRBD, passing this flag improved some
initialization time from ~5 minutes to ~5 seconds.

Signed-off-by: Lars Ellenberg <lars@linbit.com>
Acked-by: NeilBrown <neilb@suse.de>
Signed-off-by: Adrian Bunk <bunk@stusta.de>