powerpc/kasan: Limit KASAN thread size increase to 32KB

KASAN is seen to increase stack usage, to the point that it was reported
to lead to stack overflow on some 32-bit machines (see link).

To avoid overflows the stack size was doubled for KASAN builds in
commit 3e8635f ("powerpc/kasan: Force thread size increase with
KASAN").

However with a 32KB stack size to begin with, the doubling leads to a
64KB stack, which causes build errors:
  arch/powerpc/kernel/switch.S:249: Error: operand out of range (0x000000000000fe50 is not between 0xffffffffffff8000 and 0x0000000000007fff)

Although the asm could be reworked, in practice a 32KB stack seems
sufficient even for KASAN builds - the additional usage seems to be in
the 2-3KB range for a 64-bit KASAN build.

So only increase the stack for KASAN if the stack size is < 32KB.

Fixes: 18f14af ("powerpc/64s: Increase default stack size to 32KB")
Reported-by: Spoorthy <spoorthy@linux.ibm.com>
Reported-by: Benjamin Gray <bgray@linux.ibm.com>
Reviewed-by: Benjamin Gray <bgray@linux.ibm.com>
Link: https://lore.kernel.org/linuxppc-dev/bug-207129-206035@https.bugzilla.kernel.org%2F/
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20240212064244.3924505-1-mpe@ellerman.id.au

Author: mpe

arch/powerpc/include/asm/thread_info.h

@@ -14,7 +14,7 @@
 
 #ifdef __KERNEL__
 
-#ifdef CONFIG_KASAN
+#if defined(CONFIG_KASAN) && CONFIG_THREAD_SHIFT < 15
 #define MIN_THREAD_SHIFT	(CONFIG_THREAD_SHIFT + 1)
 #else
 #define MIN_THREAD_SHIFT	CONFIG_THREAD_SHIFT