reiserfs: use is_reusable to catch corruption

Build in is_reusable() unconditionally and use it to catch corruption before
it reaches the block freeing paths.

Signed-off-by: Jeff Mahoney <jeffm@suse.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>