Beginning of iovec cleanups - added two helpers (kernel_{send,recv}msg)
that do sock_sendmsg/sock_recvmsg with kvec instead of iovec; basically,
they were abstracted from earlier afs patch.  They take kvec/length of
kvec as separate arguments, do set_fs(), stick kvec into msghdr and call
sock_...msg(). 

The next group of patches will switch network filesystems to use of kvec
for kernel data + use of these helpers.  Basically, the same thing we'd
done for afs.