[PATCH] md: don't allow new md/bitmap file to be set if one already exists

... otherwise we loose a reference and can never free the file.

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>