Fix: atomic_long_add_unless() returns a boolean

Because of a documentation error in older kernels, it was assumed that
atomic_long_add_unless would return the old value, but the
implementation actually returns a boolean.

Also add missing error code int 'ret' and compare against the right type
max value.

Signed-off-by: Michael Jeanson <mjeanson@efficios.com>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>