turn do_make_slave() into transfer_propagation()

Lift calculation of replacement propagation source, removal from
peer group and assignment of ->mnt_master from do_make_slave() into
change_mnt_propagation() itself.  What remains is switching of
what used to get propagation *through* mnt to alternative source.
Rename to transfer_propagation(), passing it the replacement source
as the second argument.  Have it return void, while we are at it.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>