Add a "-f" option to pnfsdskill(8) to force disabling of a DS.

The pnfsdskill(8) command will normally fail if there is no valid mirror
for the DS to be disabled. However, a system administrator may need to
disable a DS which does not have a valid mirror so that the nfsd threads
can be terminated. This patch adds a "-f" option to pnfsdskill(8) that
uses the kernel changes made by r336141 to implement this "forced" case
of disabling a DS.
This patch only affects the pNFS server.