MD: change the parameter of md thread

Change the thread parameter, so the thread can carry extra info. Next patch
will use it.

Signed-off-by: Shaohua Li <shli@fusionio.com>
Signed-off-by: NeilBrown <neilb@suse.de>

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

diff --cc drivers/md/md.h
Simple merge

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

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

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

diff --cc drivers/md/raid5.c
Simple merge