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>