Add functions to allocate and copy into anonymous unmapped mbuf lists

[rmacklem]

Summary

The nfs-over-tls code needs to allocate ext_pgs mbufs with anonymous pages
allocated to them and copy data from a regular mbuf list into this list.
This is needed, since KERN_TLS tx only handles ext_pgs mbufs.

Since these were generic mbuf handling routines, I thought they might
have other uses, so I put them in sys/kern/kern_mbuf.c.

They could easily be renamed and moved to sys/rpc if they are felt not
to be useful elsewhere.
--> The nfs-over-tls code always calls these with M_WAITOK.

As such, maybe they shouldn't even have a "how" argument?

Test Plan

They have been tested in the nfs-over-tls code, although only
for the M_WAITOK case.

[gallatin]

kern/kern_mbuf.c

1598 ↗

(On Diff #72807)

m can be null only on the first iteration, right? I would have allocated the initial m outside the loop, parly to avoid the test on each iteration (which the compiler might optimize out), but more to reduce the cognitive load on the reader, by removing complexity from the loop.

1631 ↗

(On Diff #72807)

memcpy() seems to be preferred these days for new code

[rmacklem]

Made the changes suggested by gallatin@, which
I do think makes the code more readable.