md/raid1: update next_resync under resync_lock.

raise_barrier() uses next_resync as part of its calculations, so it
really should be updated first, instead of afterwards.

next_resync is always used under resync_lock so update it under
resync lock to, just before it is used.  That is safest.

This could cause normal IO and resync IO to interact badly so
it suitable for -stable.

Fixes: 79ef3a8a
cc: stable@vger.kernel.org (v3.13+)
Signed-off-by: NeilBrown <neilb@suse.de>