Linux 2.6.31 Compatibility Updates

SPL_AC_2ARGS_SET_FS_PWD macro updated to explicitly include
linux/fs_struct.h which was dropped from linux/sched.h.

min_wmark_pages, low_wmark_pages, high_wmark_pages macros
introduced in newer kernels.  For older kernels mm_compat.h
was introduced to define them as needed as direct mappings
to per zone min_pages, low_pages, max_pages.

module/spl/spl-kmem.c

@@ -1826,9 +1826,9 @@ spl_kmem_init_globals(void)
                 if (!populated_zone(zone))
                         continue;
 
-		minfree += zone->pages_min;
-		desfree += zone->pages_low;
-		lotsfree += zone->pages_high;
+		minfree += min_wmark_pages(zone);
+		desfree += low_wmark_pages(zone);
+		lotsfree += high_wmark_pages(zone);
 	}
 
 	/* Solaris default values */