[PATCH] md: initialise sync_blocks in raid1 resync

Otherwise it could have a random value and might BUG.  This fixes a BUG
during resync problem in raid1 introduced by the bitmap-based-intent-loggin
patches.

Signed-off-by: Neil Brown <neilb@cse.unsw.edu.au>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>