Btrfs: check to see if we have an inline item properly

If our item isn't big enough to have an actual inline item when we have skinny
metadata enabled just return 1 in find_inline_backref so we can move on to the
next item.  This probably wasn't causing a problem since we check the values of
ptr and end properly, but just in case this will keep us from doing extra work.
Thanks,

Signed-off-by: Josef Bacik <jbacik@fusionio.com>
Signed-off-by: Chris Mason <chris.mason@fusionio.com>