selftests/vm/pkeys: handle negative sys_pkey_alloc() return code

The alloc_pkey() sefltest function wraps the sys_pkey_alloc() system call.
On success, it updates its "shadow" register value because
sys_pkey_alloc() updates the real register.

But, the success check is wrong. pkey_alloc() considers any non-zero
return code to indicate success where the pkey register will be modified.
This fails to take negative return codes into account.

Consider only a positive return value as a successful call.

Link: https://lkml.kernel.org/r/[e-mail]
Fixes: 5f23f6d082a9 ("x86/pkeys: Add self-tests")
Reported-by: Thomas Gleixner <[e-mail]>
Signed-off-by: Dave Hansen <[e-mail]>
Tested-by: Aneesh Kumar K.V <[e-mail]>
Cc: Ram Pai <[e-mail]>
Cc: Sandipan Das <[e-mail]>
Cc: Florian Weimer <[e-mail]>
Cc: "Desnes A. Nunes do Rosario" <[e-mail]>
Cc: Ingo Molnar <[e-mail]>
Cc: Thiago Jung Bauermann <[e-mail]>
Cc: Michael Ellerman <[e-mail]>
Cc: Michal Hocko <[e-mail]>
Cc: Michal Suchanek <[e-mail]>
Cc: Shuah Khan <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

tools/testing/selftests/vm/protection_keys.c

···
 			" shadow: 0x%016llx\n",
 			__func__, __LINE__, ret, __read_pkey_reg(),
 			shadow_pkey_reg);
-	if (ret) {
+	if (ret > 0) {
 		/* clear both the bits: */
 		shadow_pkey_reg = set_pkey_bits(shadow_pkey_reg, ret,
 						~PKEY_MASK);