mount.nfs: force rpcbind queries if options aren't specified

mount.nfs: force rpcbind queries if options aren't	specified

nfs_options2pmap() fills in default values if the passed-in mount
options don't specify values.  This short-circuits the version, port,
and transport negotiation logic in nfs_probe_bothports().

Instead, nfs_options2pmap() should plant zeros in these pmap fields
to force nfs_probe_bothports() and nfs_advise_mount() to discover, via
rpcbind queries, what the server supports.

This fixes some scenarios where umount.nfs fails to connect to servers
that don't have all rpcbind ports open, in addition to fixing other
corner cases during mount.nfs version/protocol negotiation.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Steve Dickson <steved@redhat.com>