md: make sure MD_RECOVERY_DONE is clear before starting recovery/resync

MD_RECOVERY_DONE is normally cleared by md_check_recovery after a
resync etc finished.  However it is possible for raid5_start_reshape
to race and start a reshape before MD_RECOVERY_DONE is cleared.  This
can lean to multiple reshapes running at the same time, which isn't
good.

To make sure it is cleared before starting a reshape, and also clear
it when reaping a thread, just to be safe.

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

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

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

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