afs: Grab a reference to setp in afs_icl_Event4

afs: Grab a reference to setp in afs_icl_Event4

We can drop GLOCK in several places in afs_icl_Event4 and the
afs_icl_AppendRecord callee. To ensure that the given afs_icl_set does
not get freed while we have GLOCK dropped, grab a reference to the
set.

Thanks to Ryan C. Underwood for reporting an issue triggered by this.

Change-Id: Ifeda229b444abd75b0f22c7acf18a7553d833964
Reviewed-on: http://gerrit.openafs.org/6431
Reviewed-by: Derrick Brashear <shadow@dementix.org>
Tested-by: BuildBot <buildbot@rampaginggeek.com>