[PATCH] md: a couple of tidyups relating to the bitmap file.

1/ When init from disk, it is a BUG if there is nowhere
   to init from,
2/ use seq_path to print path in /proc/mdstat

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>