nfs: prevent assertion error with MOUNT over UDP

see http://review.gluster.org/#/c/8258/, http://review.gluster.org/8241;
and BZs 1116997, 1116503, respectively.

The MOUNT service over UDP runs in a separate thread. This thread does
not have the correct *THIS xlator set. *THIS points to the global (base)
xlator structure, but GF_CALLOC() requires it to be the NFS-xlator so
that assertions can get validated correctly.

This is solved by passing the NFS-xlator to the pthread function, and
setting the *THIS pointer explicitly in the new thread.

It seems that on occasion (needs further investigation) MOUNT over UDP
does not unregister itself. There can also be issues when the kernel NLM
implementation has been registered at portmap/rpcbind, so adding some
unregister procedures in the cleanup of the test-cases.

Change-Id: Iea3bafd65bf480f2b12da91875c73571594d52d3
BUG: 1117256
Signed-off-by: Kaleb S. KEITHLEY <kkeithle@redhat.com>
Reviewed-on: http://review.gluster.org/8262
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Niels de Vos <ndevos@redhat.com>
Reviewed-by: Santosh Pradhan <spradhan@redhat.com>