rx: Fix AIX test_and_set_bit

The AIX definition of rx_atomic_test_and_set_bit had its test the
wrong way round - so an already set bit would return false, and a clear
bit would return true. Fix this.

Change-Id: I7778c411016bca53b38257fca59ae0f7aaad64ae
Reviewed-on: http://gerrit.openafs.org/9021
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>