Move md_update_sb calls

When a change which requires a superblock update happens
at interrupt time, we currently set a flag (sb_dirty) and
wakeup to per-array thread (raid1/raid5d/multipathd) to
do the actual update.

This patch centralises this.  The sb_update is now done
by the mdrecoveryd thread.  As this is always woken up after
the error handler is called, we don't need the call to wakeup
the local thread any more.

With this, we don't need "md_update_sb" to lock the array
any more and only use __md_update_sb which is local to md.c
So we rename __md_update_sb back to md_update_sb and stop
exporting it.