x86/pkeys: Fix pkey_alloc() return value when pkeys are not supported

The man page for pkey_alloc(2) specifies that it should return -1 with
the errno set to ENOSPC when pkeys are not supported [1]. However, on
x86 pkey_alloc() sets errno to EINVAL when called for the first time
on a CPU that does not support pkeys.

The root cause of this is the x86 implementation of mm_pkey_alloc() not
directly checking if pkeys are supported. It only checks if all the
pkeys have been allocated by comparing the allocation map against
all_pkeys_mask. When OSPKE is not enabled, init_new_context() skips the
initialization of the allocation map, leaving it as 0, while
all_pkeys_mask is 1. mm_pkey_alloc() interprets this as there being a
pkey available and it returns pkey 0. Then, pkey_alloc() fails with
-EINVAL from arch_set_user_pkey_access() instead of returning -ENOSPC.
Subsequent calls to pkey_alloc() do return -ENOSPC because pkey 0 is
left marked as allocated.

Change mm_pkey_alloc() to directly check if OSPKE is enabled, and
return -1 if it is not, which causes pkey_alloc() to return -ENOSPC. The
arm64 and powerpc implementations of mm_pkey_alloc() already do this
check.

[1] https://man7.org/linux/man-pages/man2/pkey_alloc.2.html

[ dhansen: use arch_pkeys_enabled() to follow arm ]

Fixes: e8c24d3a23 ("x86/pkeys: Allocation/free syscalls")
Signed-off-by: Bijan Tabatabai <btabatabai@wisc.edu>
Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Link: https://patch.msgid.link/20260716220604.26452-1-bijan311@gmail.com

arch/x86/include/asm/pkeys.h

@@ -88,6 +88,9 @@ int mm_pkey_alloc(struct mm_struct *mm)
 	u16 all_pkeys_mask = ((1U << arch_max_pkey()) - 1);
 	int ret;
 
+	if (!arch_pkeys_enabled())
+		return -1;
+
 	/*
 	 * Are we out of pkeys?  We must handle this specially
 	 * because ffz() behavior is undefined if there are no