md: Fix regression with raid1 arrays without persistent metadata.

A RAID1 which has no persistent metadata, whether internal or
external, will hang on the first write.
This is caused by commit  070dc6dd
In that case, MD_CHANGE_PENDING never gets cleared.

So during md_update_sb, is neither persistent or external,
clear MD_CHANGE_PENDING.

This is suitable for 2.6.36-stable.

Signed-off-by: NeilBrown <neilb@suse.de>
Cc: stable@kernel.org