Add support for ext_pgs mbufs to nfsm_uiombuf().

This patch uses a slightly different algorithm for the non-ext_pgs case,
where a variable called "mcp" is maintained, pointing to the current
location that mbuf data can be filled into. This avoids use of
mtod(mp, char *) + mp->m_len to calculate the location, since this does
not work for ext_pgs mbufs and I think it makes the algorithm more readable.
This change should not result in semantic changes for the non-ext_pgs case.

This is another in the series of commits that add support to the NFS client
and server for building RPC messages in ext_pgs mbufs with anonymous pages.
This is useful so that the entire mbuf list does not need to be
copied before calling sosend() when NFS over TLS is enabled.

Since ND_EXTPG is never set yet, there is no semantic change at this time.