commit dd9ef135 upstream.

Improper arithmetics when calculting the address of the extended ref could
lead to an out of bounds memory read and kernel panic.
Signed-off-by: Quentin Casasnovas <quentin.casasnovas@oracle.com>
Reviewed-by: David Sterba <dsterba@suse.cz>
Signed-off-by: Chris Mason <clm@fb.com>
Signed-off-by: Luis Henriques <luis.henriques@canonical.com>