raid6: Move to more abstract 'ksimd' guard API

raid6: Move to more abstract 'ksimd' guard API

Move away from calling kernel_neon_begin() and kernel_neon_end()
directly, and instead, use the newly introduced scoped_ksimd() API. This
permits arm64 to modify the kernel mode NEON API without affecting code
that is shared between ARM and arm64.

Reviewed-by: Eric Biggers <ebiggers@kernel.org>
Reviewed-by: Jonathan Cameron <jonathan.cameron@huawei.com>
Acked-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>