net: skbuff.h: properly escape a macro name on kernel-doc

The "%" escape code of kernel-doc only handle letters. It
doesn't handle special chars. So, use the ``literal``
notation. That fixes this warning:

./include/linux/skbuff.h:2695: WARNING: Inline literal start-string without end-string.

No functional changes.
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>

include/linux/skbuff.h

@@ -2691,7 +2691,7 @@ bool skb_page_frag_refill(unsigned int sz, struct page_frag *pfrag, gfp_t prio);
  * @offset: the offset within the fragment (starting at the
  *          fragment's own offset)
  * @size: the number of bytes to map
- * @dir: the direction of the mapping (%PCI_DMA_*)
+ * @dir: the direction of the mapping (``PCI_DMA_*``)
  *
  * Maps the page associated with @frag to @device.
  */