If two md arrays which share real devices (i.e they each own a partition
on some device) need to sync/reconstruct at the same time, it is much
more efficient to have one wait while the other completes.

The current code uses interruptible_sleep_on which isn't SMP safe (without the BKL).
This patch re-does this code to make it more secure.  Even it two start simultaneously,
one will reliably get priority, and the other wont wait for ever.