SUNRPC: Simplify svc_unregister()

Our initial implementation of svc_unregister() assumed that PMAP_UNSET
cleared all rpcbind registrations for a [program, version] tuple.
However, we now have evidence that PMAP_UNSET clears only "inet"
entries, and not "inet6" entries, in the rpcbind database.

For backwards compatibility with the legacy portmapper, the
svc_unregister() function also must work if user space doesn't support
rpcbind version 4 at all.

Thus we'll send an rpcbind v4 UNSET, and if that fails, we'll send a
PMAP_UNSET.

This simplifies the code in svc_unregister() and provides better
backwards compatibility with legacy user space that does not support
rpcbind version 4.  We can get rid of the conditional compilation in
here as well.

This patch is part of a series that addresses
   http://bugzilla.kernel.org/show_bug.cgi?id=12256

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>