afsmonitor: Ignore return codes as appropriate

When we're not interested in the return value of a function, don't
assign it to a variable.

Caught by clang-analyzer

Change-Id: I5b576f312376780c98c2dbe4f9b85e00d529ddd8
Reviewed-on: http://gerrit.openafs.org/9185
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>