If a GFP_KERNEL allocation is attempted in md while the mddev_lock is
held, it is possible for a deadlock to eventuate.
This happens if the array was marked 'clean', and the memalloc triggers
a write-out to the md device.
For the writeout to succeed, the array must be marked 'dirty', and that
requires getting the mddev_lock.

So, before attempting a GFP_KERNEL alloction while holding the lock,
make sure the array is marked 'dirty' (unless it is currently read-only).
Signed-off-by: Neil Brown <neilb@suse.de>
Signed-off-by: Chris Wright <chrisw@sous-sol.org>