lib/crypto: gf128hash: mark clmul32() as noinline_for_stack

During randconfig testing, I came across a lot of warnings for the newly
added carryless multiplication function triggering excessive stack usage
from spilling temporary variables to the stack:

lib/crypto/gf128hash.c:166:1: error: stack frame size (1192) exceeds limit (1024) in 'polyval_mul_generic' [-Werror,-Wframe-larger-than]

In addition to the possible risk of overflowing the kernel stack,
the generated object code surely performs very poorly.

This only happens on architectures that don't provide uint128_t
(which should be all 32-bit architectures on modern compilers), but
though I tested random x86 and arm configs, I only saw this with arm's
CONFIG_THUMB2_KERNEL, which adds more pressure to the register allocator.

The testing was done using clang-22, I don't know if gcc has the same
problem. Marking clmul32() as noinline_for_stack experimentally shows
all of the affected builds to completely solve the problem, reducing
the stack usage to a few bytes as expected.

Since u64 arithmetic frequently leads to compilers badly optimizing
32-bit targets, keeping clmul32 out of line is likely to help on
other 32-bit configurations as well when they run into this problem,
though it may also result in a small performance degradation in
configurations that would benefit from inlining.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Link: https://patch.msgid.link/20260611125952.3387258-1-arnd@kernel.org
Signed-off-by: Eric Biggers <ebiggers@kernel.org>

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