allocation, as required.
If m_getm() receives NULL as a first argument, then it allocates `len'
(second argument) bytes worth of mbufs + clusters and returns the chain
only if it was able to allocate everything.
If the first argument is non-NULL, then it should be an existing mbuf
chain (e.g. pre-allocated mbuf sitting on a ring, on some list, etc.) and
so it will allocate `len' bytes worth of clusters and mbufs, as needed,
and append them to the tail of the passed in chain, only if it was able
to allocate everything requested.
If allocation fails, only what was allocated by the routine will be freed,
and NULL will be returned.
Also, get rid of existing m_getm() in netncp code and replace calls to it
to calls to this new generic code.
Heavily Reviewed by: bp
fd2da3a771