it's an error to set the buffer bytecount to anything larger than
the buffer's original allocation size, but anything less than that
is ok.
Also, in ndis_ptom(), use the same logic: if the bytecount is
larger than the allocation size, consider the bytecount invalid
and the allocation size as the packet fragment length (m_len)
instead of the bytecount.
This corrects a consistency problem between the Broadcom wireless
driver and some of the ethernet drivers I've tested: the ethernet
drivers all report the packet frag sizes in buf->nb_bytecount, but
the Broadcom wireless driver reports them in buf->nb_size. This
seems like a bug to me, but it clearly must work in Windows, so
we have to deal with it here too.
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