md: perform async updates for metadata where possible.

When adding devices to, or removing device from, an array we need to
update the metadata.  However we don't need to do it synchronously as
data integrity doesn't depend on these changes being recorded
instantly.  So avoid the synchronous call to md_update_sb and just set
a flag so that the thread will do it.

This can reduce the number of updates performed when lots of devices
are being added or removed.

Signed-off-by: NeilBrown <neilb@suse.com>
Signed-off-by: Shaohua Li <shli@fb.com>