selftests/mm: Define PKEY_UNRESTRICTED for pkey_sighandler_tests

Commit 6e182dc9f268 ("selftests/mm: Use generic pkey register
manipulation") makes use of PKEY_UNRESTRICTED in
pkey_sighandler_tests. The macro has been proposed for addition to
uapi headers [1], but the patch hasn't landed yet.

Define PKEY_UNRESTRICTED in pkey-helpers.h for the time being to fix
the build.

[1] https://lore.kernel.org/all/[e-mail]/

Fixes: 6e182dc9f268 ("selftests/mm: Use generic pkey register manipulation")
Reported-by: Aishwarya TCV <[e-mail]>
Signed-off-by: Kevin Brodsky <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Catalin Marinas <[e-mail]>

tools/testing/selftests/mm/pkey-helpers.h

···
 #define PKEY_MASK	(PKEY_DISABLE_ACCESS | PKEY_DISABLE_WRITE)
 #endif
 
+/*
+ * FIXME: Remove once the generic PKEY_UNRESTRICTED definition is merged.
+ */
+#ifndef PKEY_UNRESTRICTED
+#define PKEY_UNRESTRICTED 0x0
+#endif
+
 #ifndef set_pkey_bits
 static inline u64 set_pkey_bits(u64 reg, int pkey, u64 flags)
 {