Btrfs: fixup error path in __btrfs_inc_extent_ref

When we fail to add a reference after a non-inline insertion by some reasons,
eg. ENOSPC, we'll abort the transaction, but we don't return this error to
the caller who has to walk around again to find something wrong, that's
unnecessary.

Also fixup other error paths to keep it simple.

Signed-off-by: Liu Bo <bo.li.liu@oracle.com>
Signed-off-by: Josef Bacik <jbacik@fusionio.com>
Signed-off-by: Chris Mason <chris.mason@fusionio.com>