md: remove typedefs: mdk_thread_t -> struct md_thread

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/raid1.h
Simple merge

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

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