NFS: tidy up nfs_show_mountd_netid

This function is a bit clumsy, incorrectly producing
",mountproto=" if mountd_protocol is 0 and !showdefaults,
and duplicating the code for reporting "auto".

Tidy it up so that it only makes a single seq_printf() call,
and more obviously does the right thing.

Fixes: ee671b01 ("NFS: convert proto= option to use netids rather than a protoname")
Signed-off-by: NeilBrown <neilb@suse.com>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>