[PATCH] dm mirror log: sync_count fix

When a mirror is reduced in size, clear the part of the bitmap that is no
longer used.

Signed-off-by: Alasdair G Kergon <agk@redhat.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>