rx: Some kernels have no reschedule function

rx: Some kernels have no reschedule function

If RXK_TIMEDSLEEP_ENV isn't set, then Unix kernel cache managers
call rxevent_Init without a reschedule function. Check for this so
we don't end up calling a NULL function in these situations.

Change-Id: I5e89f5247aeffc4c27d3f81c0ccabe4979232846
Reviewed-on: http://gerrit.openafs.org/6206
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Tested-by: Benjamin Kaduk <kaduk@mit.edu>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@secure-endpoints.com>