sh: Use the atomic_t "counter" member

Now that atomic_t is a generic opaque type for all architectures, it is
unwise to use intimate knowledge of its internals when manipulating it.

Instead of relying on the "counter" member being at offset 0 from the
beginning of an atomic_t, explicitly reference the member. This guards
us from any changes to the layout of the beginning of the atomic_t type.

Signed-off-by: Matt Fleming <matt@console-pimps.org>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>