Add iob_ensure_headroom() placeholder.

src/include/gpxe/iobuf.h

 
 #include <stdint.h>
 #include <assert.h>
+#include <errno.h>
 #include <gpxe/list.h>
 
 /**
 	return ( iobuf->end - iobuf->tail );
 }
 
+/**
+ * Ensure I/O buffer has sufficient headroom
+ *
+ * @v iobuf	I/O buffer
+ * @v len	Required headroom
+ *
+ * This function currently only checks for the required headroom; it
+ * does not reallocate the I/O buffer if required.  If we ever have a
+ * code path that requires this functionality, it's a fairly trivial
+ * change to make.
+ */
+static inline __attribute__ (( always_inline )) int
+iob_ensure_headroom ( struct io_buffer *iobuf, size_t len ) {
+	if ( iob_headroom ( iobuf ) >= len )
+		return 0;
+	return -ENOBUFS;
+}
+
 extern struct io_buffer * alloc_iob ( size_t len );
 extern void free_iob ( struct io_buffer *iobuf );
 extern void iob_pad ( struct io_buffer *iobuf, size_t min_len );