base: genlock: Avoid a race condition when releasing locks

Avoid a race condition if a lock gets released by all owners
but a process fails to close a lock file descriptor and tries
to reuse it.  Clearing the pointers to the data will ensure
that attaching a dead lock will return an error.

Change-Id: Ic0dedbadd693c7a22b027052cdd247370b28a7c5
Signed-off-by: Jordan Crouse <jcrouse@codeaurora.org>