linux: rpm: Fix SELinux attributes on /afs when installing openafs-client package

linux: rpm: Fix SELinux attributes on /afs when installing openafs-client package

Since the directory /afs isn't included in the package manifest, but
rather created in a script in the openafs-client package, it never
gets the appropriate SELinux attributes that are required to mount a
volume (mnt_t).

This change fixes the problem by running '/sbin/restorecon' (if it is
an executable that exists) on the /afs directory after the
openafs-client package is installed, right after the directory is
created.

Change-Id: I3905cf8da8f7414e410acfa8df51b35abe057cd7
Reviewed-on: http://gerrit.openafs.org/4763
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Simon Wilkinson <sxw@inf.ed.ac.uk>
Reviewed-by: Derrick Brashear <shadow@dementia.org>