UPSTREAM: crypto: arm/chacha - fix build failured when kernel mode NEON is disabled

When the ARM accelerated ChaCha driver is built as part of a configuration
that has kernel mode NEON disabled, we expect the compiler to propagate
the build time constant expression IS_ENABLED(CONFIG_KERNEL_MODE_NEON) in
a way that eliminates all the cross-object references to the actual NEON
routines, which allows the chacha-neon-core.o object to be omitted from
the build entirely.

Unfortunately, this fails to work as expected in some cases, and we may
end up with a build error such as

  chacha-glue.c:(.text+0xc0): undefined reference to `chacha_4block_xor_neon'

caused by the fact that chacha_doneon() has not been eliminated from the
object code, even though it will never be called in practice.

Let's fix this by adding some IS_ENABLED(CONFIG_KERNEL_MODE_NEON) tests
that are not strictly needed from a logical point of view, but should
help the compiler infer that the NEON code paths are unreachable in
those cases.

Fixes: b36d8c09e710c71f ("crypto: arm/chacha - remove dependency on generic ...")
Reported-by: Russell King <linux@armlinux.org.uk>
Cc: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
(cherry picked from commit 0bc81767c5bd9d005fae1099fb39eb3688370cb1)
Bug: 152722841
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>
Change-Id: I073e88ef1ebcf2c1cd0cf3ac75833bb043e18649