x86/pkeys/selftests: Fix pkey exhaustion test off-by-one

In our "exhaust all pkeys" test, we make sure that there
is the expected number available.  Turns out that the
test did not cover the execute-only key, but discussed
it anyway.  It did *not* discuss the test-allocated
key.

Now that we have a test for the mprotect(PROT_EXEC) case,
this off-by-one issue showed itself.  Correct the off-by-
one and add the explanation for the case we missed.

Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Dave Hansen <dave.hansen@intel.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Michael Ellermen <mpe@ellerman.id.au>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Ram Pai <linuxram@us.ibm.com>
Cc: Shuah Khan <shuah@kernel.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: linux-mm@kvack.org
Link: http://lkml.kernel.org/r/20180509171350.E1656B95@viggo.jf.intel.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>

tools/testing/selftests/x86/protection_keys.c

@@ -1163,12 +1163,15 @@ void test_pkey_alloc_exhaust(int *ptr, u16 pkey)
 	pkey_assert(i < NR_PKEYS*2);
 
 	/*
-	 * There are 16 pkeys supported in hardware.  One is taken
-	 * up for the default (0) and another can be taken up by
-	 * an execute-only mapping.  Ensure that we can allocate
-	 * at least 14 (16-2).
+	 * There are 16 pkeys supported in hardware.  Three are
+	 * allocated by the time we get here:
+	 *   1. The default key (0)
+	 *   2. One possibly consumed by an execute-only mapping.
+	 *   3. One allocated by the test code and passed in via
+	 *      'pkey' to this function.
+	 * Ensure that we can allocate at least another 13 (16-3).
 	 */
-	pkey_assert(i >= NR_PKEYS-2);
+	pkey_assert(i >= NR_PKEYS-3);
 
 	for (i = 0; i < nr_allocated_pkeys; i++) {
 		err = sys_pkey_free(allocated_pkeys[i]);