md: export various start/stop interfaces

export entry points for starting and stopping md arrays.
This will be used by a module to make md/raid5 work under
dm.
Also stop calling md_stop_writes from md_stop, as that won't
work well with dm - it will want to call the two separately.

Signed-off-by: NeilBrown <neilb@suse.de>

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

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