Btrfs: Change btrfs_truncate_inode_items to stop when it hits the inode

btrfs_truncate_inode_items is setup to stop doing btree searches when
it has finished removing the items for the inode.  It used to detect the
end of the inode by looking for an objectid that didn't match the
one we were searching for.

But, this would result in an extra search through the btree, which
adds extra balancing and cow costs to the operation.

This commit adds a check to see if we found the inode item, which means
we can stop searching early.

Signed-off-by: Chris Mason <chris.mason@oracle.com>