slab: Decouple slab_debug and no_hash_pointers

Some system owners use slab_debug=FPZ (or similar) as a hardening option,
but do not want to be forced into having kernel addresses exposed due
to the implicit "no_hash_pointers" boot param setting.[1]

Introduce the "hash_pointers" boot param, which defaults to "auto"
(the current behavior), but also includes "always" (forcing on hashing
even when "slab_debug=..." is defined), and "never". The existing
"no_hash_pointers" boot param becomes an alias for "hash_pointers=never".

This makes it possible to boot with "slab_debug=FPZ hash_pointers=always".

Link: https://github.com/KSPP/linux/issues/368 [1]
Fixes: 792702911f ("slub: force on no_hash_pointers when slub_debug is enabled")
Co-developed-by: Sergio Perez Gonzalez <sperezglz@gmail.com>
Signed-off-by: Sergio Perez Gonzalez <sperezglz@gmail.com>
Acked-by: Vlastimil Babka <vbabka@suse.cz>
Acked-by: David Rientjes <rientjes@google.com>
Reviewed-by: Bagas Sanjaya <bagasdotme@gmail.com>
Signed-off-by: Kees Cook <kees@kernel.org>
Reviewed-by: Harry Yoo <harry.yoo@oracle.com>
Acked-by: Rafael Aquini <raquini@redhat.com>
Tested-by: Petr Mladek <pmladek@suse.com>
Reviewed-by: Petr Mladek <pmladek@suse.com>
Link: https://patch.msgid.link/20250415170232.it.467-kees@kernel.org
[kees@kernel.org: Add note about hash_pointers into slab_debug kernel parameter documentation.]
Signed-off-by: Petr Mladek <pmladek@suse.com>

include/linux/sprintf.h

@@ -22,7 +22,7 @@ __scanf(2, 0) int vsscanf(const char *, const char *, va_list);
 
 /* These are for specific cases, do not use without real need */
 extern bool no_hash_pointers;
-int no_hash_pointers_enable(char *str);
+void hash_pointers_finalize(bool slub_debug);
 
 /* Used for Rust formatting ('%pA') */
 char *rust_fmt_argument(char *buf, char *end, const void *ptr);