net: fix warnings in 'make htmldocs' by moving macro definition out of field declaration

commit 7bbadd2d upstream.

Docbook does not like the definition of macros inside a field declaration
and adds a warning. Move the definition out.

Fixes: 79462ad0 ("net: add validation for the socket syscall protocol argument")
Reported-by: kbuild test robot <lkp@intel.com>
Signed-off-by: Hannes Frederic Sowa <hannes@stressinduktion.org>
Signed-off-by: David S. Miller <davem@davemloft.net>
[bwh: Backported to 3.2: keep open-coding U8_MAX]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>