buffers: make buffer data size configurable from startup config

Example:

buffers {
	default data-size 1536
}

Change-Id: I5b4436850ca18025c9fdcfc7ed648c2c2732d660
Signed-off-by: Damjan Marion <damarion@cisco.com>
diff --git a/src/vnet/sctp/sctp.c b/src/vnet/sctp/sctp.c
index d0e28c5..21cc78b 100644
--- a/src/vnet/sctp/sctp.c
+++ b/src/vnet/sctp/sctp.c
@@ -892,7 +892,7 @@
   vec_validate (tm->ip_lookup_tx_frames[0], num_threads - 1);
   vec_validate (tm->ip_lookup_tx_frames[1], num_threads - 1);
 
-  tm->bytes_per_buffer = VLIB_BUFFER_DATA_SIZE;
+  tm->bytes_per_buffer = vlib_bufer_get_default_size (vm);
 
   vec_validate (tm->time_now, num_threads - 1);
   return error;