Grow: update backup-metadata mtime every time we write it.

Originally the backup-metadata was only written once at the
start of a raid5 reshape that made the array bigger.  So we only
set the mtime once.

Now that we can be writing metadata continually during an in-place
reshape, we need to update the mtime more often.

Also, allow the metadata mtime to be slightly in advance of the
array mtime.  Normally the difference will be less than a second,
so 10 minutes should be plenty.  This guards against an old backup
file being used to restart an array.  but starting two reshapes in the
10 minutes is sufficiently unlikely, and the possibility of an
accident is already sufficiently small, that 10 minutes is probably
fine.

Thanks to Guy Martin <gmsoft@tuxicoman.be> for discovering and
reporting that .mtime wasn't being updated properly.

Signed-off-by: NeilBrown <neilb@suse.de>

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