reiserfs: fix usage of signed ints for block numbers

Do a quick signedness check for block numbers.  There are a number of places
where signed integers are used for block numbers, which limits the usable file
system size to 8 TiB.  The disk format, excepting a problem which will be
fixed in the following patch, supports file systems up to 16 TiB in size.
This patch cleans up those sites so that we can enable the full usable size.

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>