rx: Raise minimum Linux atomics version to 2.6

rx: Raise minimum Linux atomics version to 2.6

Linux 2.4 does not have atomic_dec_return. If we switch to a
dec_and_test-like API, then we could use the Linux 2.4 atomics. But
for now, just raise the minimum to 2.6, and for 2.4 and below just use
the generic atomics implementation so we can build.

Change-Id: Ie584bd420839dbcbe3158fe7b50df633cb7eba6d
Reviewed-on: http://gerrit.openafs.org/7875
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementix.org>