libafscp: Can't unlock something we've freed

When we call _StatCleanup on a stored statent structure, it
deletes the mutex, and frees the structure itself. This means it
can't be called with a locked structure as the mutex deletion
will fail, and then we'll try to reference freed memory when we
later unlock that mutex.

Fix this by unlocking the mutex before calling _StatCleanup. This
is safe because the only reference to the structure visible to other
threads must have been deleted by the time we reach this point.

Caught by coverity (#986058, #986059)

Change-Id: I346d4c8a7cd478db044af919662c1cf1c093e205
Reviewed-on: http://gerrit.openafs.org/9297
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>