umount.nfs: move nfs_call_umount to network.c

nfs_call_umount() is shared by nfsmount.c and nfsumount.c, and manages a
network function (building the RPC umount call to the server's MNT daemon).
So move it to network.c with other network-related functions.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Neil Brown <neilb@suse.de>