SUNRPC: Use short-hand IPv6 ANYADDR for RPCB_SET

Clean up: When doing an RPCB_SET, make the kernel's rpcb client use the
shorthand "::" for the universal form of the IPv6 ANY address.

Without this patch, rpcbind will advertise:

  0000:0000:0000:0000:0000:0000:0000:0000.x.y

This is cosmetic only.  It cleans up the display of information from
/sbin/rpcinfo.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: J. Bruce Fields <bfields@citi.umich.edu>