linux.h: don't redefine lower_32_bits if already defined

systrace.c fails to build if we're using a common compiler.h for both
openzfs and linuxkpi. The issue is easy enough to fix: don't redefined
lower_32_bits if it's already defined in linux.h, since it's the least
'standardized'. This will allow systrace.c to build using an equivalent
macro.

MFC After: 3 days
Sponsored by: Netflix