Strengthen the locking of mddev.

mddev is only ever locked in md.c, so we move {,un}lock_mddev
out of the header and into md.c, and rename to mddev_{,un}lock
for consistancy with mddev_{get,put,find}.

When building arrays (typically at boot time) we now lock, and unlock
as it is the "right" thing to do.  The lock should never fail.

When generating /proc/mdstat, we lock each array before inspecting it.

In md_ioctl, we lock the mddev early and unlock at the end, rather than
locking in two different places.

In md_open we make sure we can get a lock before completing the open.  This
ensures that we sync with do_md_stop properly.

In md_do_recovery, we lock each mddev before checking it's status.

md_do_recovery must unlock while recovery happens, and a do_md_stop at this
point will deadlock when md_do_recovery tries to regain the lock.  This will be
fixed in a later patch.