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

sys/compat/linux/linux.h

@@ -350,7 +350,9 @@ struct l_statx {
 #define	LINUX_ST_RELATIME		0x1000	/* No native analogue */
 #define	LINUX_ST_NOSYMFOLLOW		0x2000
 
+#ifndef lower_32_bits
 #define	lower_32_bits(n)	((uint32_t)((n) & 0xffffffff))
+#endif
 
 #ifdef KTRACE
 #define	linux_ktrsigset(s, l)	\