if_setrdomain could potentially call if_clone_create recursively in the create

if_setrdomain could potentially call if_clone_create recursively in the create
rdomain case leading to locking issues and lots of headscratching. turns out
the only case where if_setrdomain could actually create an rdomain and thus
end up with that pattern is the ioctl path.
make if_setrdomain never create an rdomain, return error if it doesn't exist
already, introduce if_createrdomain, and adjust the ioctl path to use it.
ok sashan bluhm claudio