nfs: prevent assertion error with MOUNT over UDP

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.

Cherry picked from commit ec74ceedaa41047b88d270c00eeb071b73e19664:
> Change-Id: I3be5a420fc800bbcc14198d0b6faf4cf2c7300b1
> BUG: 1116503
> Signed-off-by: Niels de Vos <ndevos@redhat.com>
> Reviewed-on: http://review.gluster.org/8241
> Reviewed-by: Santosh Pradhan <spradhan@redhat.com>
> Tested-by: Gluster Build System <jenkins@build.gluster.com>
> Reviewed-by: Vijay Bellur <vbellur@redhat.com>

Change-Id: I3be5a420fc800bbcc14198d0b6faf4cf2c7300b1
BUG: 1116997
Signed-off-by: Niels de Vos <ndevos@redhat.com>
Reviewed-on: http://review.gluster.org/8258
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Kaleb KEITHLEY <kkeithle@redhat.com>
Reviewed-by: Santosh Pradhan <spradhan@redhat.com>