cluster/afr: Avoid self-healing extended attribute used by SELinux.

Since removexattr() fails to remove "security.selinux" in a system 
where SELinux is enforcing, xattr self-healing fails. 
As a consequence of this, user extended attributes are not being healed.
Added a check in afr to prune SELinux xattr from the dictionary 
used for removing xattrs from the sink.

Minor changes in tests and md-cache as well.

Signed-off-by: Vijay Bellur <vbellur@redhat.com>
Change-Id: I854bfc0098dde812ce2afe64b125ee40c04bdeb1
BUG: 957877
Reviewed-on: http://review.gluster.org/4905
Reviewed-by: Venky Shankar <vshankar@redhat.com>
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Anand Avati <avati@redhat.com>
Fix uninitialized mutex usage in synctask_destroy

synctask_new() initialize task->mutex is task->synccbk is NULL.
synctask_done() calls synctask_destroy() if task->synccbk is not NULL.
synctask_destroy() always destroys the mutex.

Fix that by checking for task->synccbk in synctask_destroy()

BUG: 764655
Change-Id: I50bb53bc6e2738dc0aa830adc4c1ea37b24ee2a0
Signed-off-by: Emmanuel Dreyfus <manu@netbsd.org>
Reviewed-on: http://review.gluster.org/4913
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Anand Avati <avati@redhat.com>