libmctp: Split padding in pktbuf into header and trailer

The astlpc binding will shortly make use of a medium-specific trailer to
implement a CRC-32 integrity check over packet data. Allow for
medium-specific trailer data in the pktbuf in order to remove the need
for a bounce-buffer. The binding will trim the pktbuf to the payload
length before passing the instance up the stack.

Signed-off-by: Andrew Jeffery <andrew@aj.id.au>
Change-Id: Ib215a552ff218b007f7f119c5235fbc75d9ca6cb
diff --git a/core.c b/core.c
index 7b1d072..19202e8 100644
--- a/core.c
+++ b/core.c
@@ -90,13 +90,13 @@
 	struct mctp_pktbuf *buf;
 	size_t size;
 
-	size = binding->pkt_size + binding->pkt_pad;
+	size = binding->pkt_size + binding->pkt_header + binding->pkt_trailer;
 
 	/* todo: pools */
 	buf = __mctp_alloc(sizeof(*buf) + size);
 
 	buf->size = size;
-	buf->start = binding->pkt_pad;
+	buf->start = binding->pkt_header;
 	buf->end = buf->start + len;
 	buf->mctp_hdr_off = buf->start;
 	buf->next = NULL;