Replace obscure constructs in fs/block_dev.c

Replace some funky codepaths in fs/block_dev.c with cleaner versions of the
affected places.

[akpm@linux-foundation.org: fix return value]
Signed-off-by: Johannes Weiner <hannes-kernel@saeurebad.de>
Cc: Bjorn Steinbrink <B.Steinbrink@gmx.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>