loopdev: ignore negative offset when calculate loop device size

loopdev: ignore negative offset when calculate loop device size

Negative offset may cause loop device size larger than backing file
size.

 $ fallocate -l 1M a
 $ losetup --offset 0xffffffffffff0000 /dev/loop0 a
 $ blockdev --getsize64 /dev/loop0
 1114112
 $ ls -l a
 -rw-r--r-- 1 root root 1048576 Jan 23 12:46 a
 $ cat /dev/loop0
 cat: /dev/loop0: Input/output error

It makes no sense to do that. Only apply offset when it's positive.

Fix a typo in the comment by the way.

Signed-off-by: Guo Chao <yan@linux.vnet.ibm.com>
Cc: Alexander Viro <viro@zeniv.linux.org.uk>
Cc: Guo Chao <yan@linux.vnet.ibm.com>
Cc: M. Hindess <hindessm@uk.ibm.com>
Cc: Nikanth Karthikesan <knikanth@suse.de>
Cc: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>