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: Nkbuild test robot <lkp@intel.com>
Signed-off-by: NHannes Frederic Sowa <hannes@stressinduktion.org>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>