Add linux compatibility tests

While the splat tests were originally designed to stress test
the Solaris primatives.  I am extending them to include some kernel
compatibility tests.  Certain linux APIs have changed frequently.
These tests ensure that added compatibility is working properly
and no unnoticed regression have slipped in.

Test 1 and 2 add basic regression tests for shrink_icache_memory
and shrink_dcache_memory.  These are simply functional tests to
ensure we can call these functions safely.  Checking for correct
behavior is more difficult since other running processes will
influence the behavior.  However, these functions are provided
by the kernel so if we can successfully call them we assume they
are working correctly.

Test 3 checks that shrinker functions are being registered and
called correctly.  As of Linux 3.0 the shrinker API has changed
four different times so I felt the need to add a trivial test
case to ensure each variant works as expected.

include/splat-ctl.h

@@ -105,6 +105,7 @@ typedef struct splat_cmd {
 #define SPLAT_SUBSYSTEM_GENERIC		0x0d00
 #define SPLAT_SUBSYSTEM_CRED		0x0e00
 #define SPLAT_SUBSYSTEM_ZLIB		0x0f00
+#define SPLAT_SUBSYSTEM_LINUX		0x1000
 #define SPLAT_SUBSYSTEM_UNKNOWN		0xff00
 
 #endif /* _SPLAT_CTL_H */