LinuxKPI: correct the LINUXKPI_VERSION check for abs_diff()

The code introduced abs_diff() as migrated from drm to standard
linux headers.  The LINUXKPI_VERSION check did not allow this to
be visible by default in-tree.  Make sure the v6.6 check is optional.
drm-kmod always defines LINUXKPI_VERSION so this should be fine.

Sponsored by:	The FreeBSD Foundation (initially)
Fixes:		afc450f
MFC after:	3 days
Reviewed by:	dumbbell
Differential Revision:	https://reviews.freebsd.org/D52071

Author: Bjoern A. Zeeb

sys/compat/linuxkpi/common/include/linux/math.h

@@ -56,7 +56,7 @@
 	__ret;						\
 })
 
-#if defined(LINUXKPI_VERSION) && LINUXKPI_VERSION >= 60600
+#if !defined(LINUXKPI_VERSION) || (LINUXKPI_VERSION >= 60600)
 #define abs_diff(x, y) ({		\
 	__typeof(x) _x = (x);		\
 	__typeof(y) _y = (y);		\