features/changelog: prevent deadlock on thread cancellation

helper threads (fsync, rollover) wake up periodically and perform
their respective operation under a lock (crt->lock). These threads
are also subjected to cancellation under some circumstance such as
disabling changelog. This is inherently dangerous when funtions
which are cancellation points for pthread_cancel(3) are used
in the locked region.

Consider this

         pthread_mutex_lock(&mutex);
         {
                /* ... */
                ret = fsync (fd);   <-- cancellation point
                /* ... */
         }
         pthread_mutex_unlock(&mutex);

A pthread_cancel(3) by another thread just before fsync(3) but
after pthread_mutex_lock(3) would result in the thread getting
cancelled when fsync(3) is invoked, thereby never unlocking the
mutex. Moreover, in case of changelog translator, the locked
region (under crt->lock in changelog-rt.c) is also the code
path for fop changelog updation. Therefore, unlocking the
mutex in thread cleanup handler (pthread_cleanup_pop(3)) might
prematurely release the mutex during fop updation path.

This patch fixes such problems existing in fsync and rollover
threads. Fix is to enter the locked region with cancellation
disabled and enable it after mutex unlock. Also, test for a
cancellation request early on in case none of the functions
are cancellation points.

Change-Id: I1795627a12827609c1da659d07fc1457ffa033de
BUG: 1110917
Signed-off-by: Venky Shankar <vshankar@redhat.com>
Reviewed-on: http://review.gluster.org/8106
Reviewed-by: Kotresh HR <khiremat@redhat.com>
Reviewed-by: Vijay Bellur <vbellur@redhat.com>
Tested-by: Gluster Build System <jenkins@build.gluster.com>