process-util: use only the least significant byte from personality()

The personality() syscall returns a 32-bit value where the top three
bytes are reserved for flags that emulate historical or architectural
quirks, and only the least significant byte reflects the actual
personality we're interested in (in opinionated_personality()).

Use the newly defined mask in the corresponding test as well, otherwise
the test fails on some more "exotic" architectures that set some of the
"quirk" flags:

~# uname -m
armv7l
~# build/test-seccomp
...
/* test_lock_personality */
current personality=0x0
safe_personality(PERSONALITY_INVALID)=0x800000
Assertion '(unsigned long) safe_personality(current) == current' failed at src/test/test-seccomp.c:970, function test_lock_personality(). Aborting.
lockpersonalityseccomp terminated by signal ABRT.
Assertion 'wait_for_terminate_and_check("lockpersonalityseccomp", pid, WAIT_LOG) == EXIT_SUCCESS' failed at src/test/test-seccomp.c:996, function test_lock_personality(). Aborting.
Aborted (core dumped)

See: personality(2) and comments in sys/personality.h

src/basic/process-util.c

@@ -1303,7 +1303,7 @@ int opinionated_personality(unsigned long *ret) {
         if (current < 0)
                 return current;
 
-        if (((unsigned long) current & 0xffff) == PER_LINUX32)
+        if (((unsigned long) current & OPINIONATED_PERSONALITY_MASK) == PER_LINUX32)
                 *ret = PER_LINUX32;
         else
                 *ret = PER_LINUX;

src/basic/process-util.h

@@ -107,6 +107,11 @@ bool oom_score_adjust_is_valid(int oa);
 #define PERSONALITY_INVALID 0xffffffffLU
 #endif
 
+/* The personality() syscall returns a 32-bit value where the top three bytes are reserved for flags that
+ * emulate historical or architectural quirks, and only the least significant byte reflects the actual
+ * personality we're interested in. */
+#define OPINIONATED_PERSONALITY_MASK 0xFFUL
+
 unsigned long personality_from_string(const char *p);
 const char *personality_to_string(unsigned long);