[PATCH] md: bio leak fix

insert a missing bio_put when writting the md superblock.

Without this we have a steady growth in the "bio" slab.

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>