common: do not unlock rwlock on destruction

According to pthread_rwlock_unlock(3p):

 Results are undefined if the read-write lock rwlock is not held
 by the calling thread.

and:

 https://sourceware.org/bugzilla/show_bug.cgi?id=17561

 Calling pthread_rwlock_unlock on an rwlock which is not locked
 is undefined.

calling pthread_rwlock_unlock on RWLock destruction could cause
an unknown behavior for two reasons:

- the lock is acquired by another thread (undefined)
- the lock is not acquired (undefined)

Moreover since glibc-2.20 calling pthread_rwlock_unlock on a
rwlock that is not locked results in a SIGILL that kills the
application.

This patch removes the pthread_rwlock_unlock call on destruction
and replaces it with an assertion to check that the RWLock is
not in use.

Any code that relied on the implicit release is now going to
break the assertion, e.g.:

 {
   RWLock l;
   l.get(for_write);
 } // implicit release, wrong.

Signed-off-by: Federico Simoncelli <fsimonce@redhat.com>
Merge pull request #2937 from simon3z/master

common: do not unlock rwlock on destruction

Reviewed-by: Samuel Just <sjust@redhat.com>
Reviewed-by: Sage Weil <sage@redhat.com>