md: give userspace control over removing failed devices when external metdata in use

When a device fails, we must not allow an further writes to the array until
the device failure has been recorded in array metadata.  When metadata is
managed externally, this requires some synchronisation...

Allow/require userspace to explicitly remove failed devices from active
service in the array by writing 'none' to the 'slot' attribute.  If this
reduces the number of failed devices to 0, the write block will automatically
be lowered.

Signed-off-by: Neil Brown <neilb@suse.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>