SUNRPC: Use rpc_pton() in ip_map_parse()

The existing logic in ip_map_parse() can not currently parse
shorthanded IPv6 addresses (anything with a double colon), nor can
it parse an IPv6 presentation address with a scope ID.  An
IPv6-enabled mountd can pass down both.

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