md: make sure a reshape is started when device switches to read-write

md: make sure a reshape is started when device switches to read-write

A resync/reshape/recovery thread will refuse to progress when the array is
marked read-only.  So whenever it mark it not read-only, it is important to
wake up thread resync thread.  There is one place we didn't do this.

The problem manifests if the start_ro module parameters is set, and a raid5
array that is in the middle of a reshape (restripe) is started.  The array
will initially be semi-read-only (meaning it acts like it is readonly until
the first write).  So the reshape will not proceed.

On the first write, the array will become read-write, but the reshape will not
be started, and there is no event which will ever restart that thread.

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>