rx: Add the add_and_read function to our atomics

rx: Add the add_and_read function to our atomics

Add rx_atomic_add_and_read, which lets us get the new value out
of an atomic addition operation.

Change-Id: If74b1357390329b8c59042d3154491a103fe0d6c
Reviewed-on: http://gerrit.openafs.org/7974
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementix.org>