block: Add submit_bio_wait(), remove from md

Random cleanup - this code was duplicated and it's not really specific
to md.

Also added the ability to return the actual error code.

Signed-off-by: Kent Overstreet <koverstreet@google.com>
CC: Jens Axboe <axboe@kernel.dk>
CC: NeilBrown <neilb@suse.de>
Acked-by: Tejun Heo <tj@kernel.org>

diff --cc drivers/md/raid1.c
Simple merge

diff --cc drivers/md/raid10.c
Simple merge

diff --cc fs/bio.c
Simple merge

diff --cc include/linux/bio.h
Simple merge