Remove 0 filling from nfsm_uiombuflist().

nfsm_uiombuflist() zero filled the mbuf list to a multiple of 4bytes
as required for XDR. Unfortunately that modified an mbuf list after
it was m_copym()'d and was broken. This patch removes the zero filling code.
Since nfsm_uiombuflist() is not yet used in head/current, this has no
effect on users.
The function will be used by a future commit of code that adds Flex
File Layout support.