md_check_recovery only locks a device and does stuff when it thinks there is a
real likelyhood that something needs doing.  So the test at the top must cover
all possibilities.

But it didn't cover the possibility that the last outstanding request on a
failed device had finished and so the device needed to be removed.

As a result, a failed drive might not get removed from the personalities
perspective on the array, and so it could never be removed from the array as a
whole.

With this patch, whenever ->nr_pending hits zero on a faulty device,
MD_RECOVERY_NEEDED is set so that md_check_recovery will do stuff.
Signed-off-by: Neil Brown <neilb@cse.unsw.edu.au>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>