lib: Move SipHash into lib/crypto/

The addition of SipHash to lib/ predated the existence of the
lib/crypto/ directory.  Since SipHash is a cryptographic algorithm, move
it from the top-level lib/ directory to lib/crypto/.

(This does mean HalfSipHash comes with it too, as it's in the same
files.  But lib/crypto/ still seems like the proper place for both.)

Specifically:

- Move both siphash.c and siphash_kunit.c.  Keep the header
  <linux/siphash.h> as-is for now.

- Drop the dedicated MAINTAINERS entry for SipHash, as per the
  suggestion from Jason
  (https://lore.kernel.org/r/asB7QVFMlgKgbje2@zx2c4.com/).  The C files
  now just fold up to the "CRYPTO LIBRARY" entry.  Add the header
  explicitly, as it otherwise would be left unmaintained.

- Move the kconfig option that controls the KUnit test.  Put
  "CRYPTO_LIB" in its name and update the prompt and help text to make
  it consistent with the other crypto library test options.

- Enable the KUnit test in lib/crypto/.kunitconfig.

- Link to siphash.rst from the appropriate place in libcrypto-hash.rst.

- Use obj-y instead of lib-y, for consistency with the fact that
  currently lib/crypto/ doesn't use kbuild's support for library file
  goals.  We could keep it as lib-y, but virtually every kernel needs
  SipHash support anyway (with vsprintf, printk, IPv4, IPv6, etc.
  depending on it) so there is no practical difference in this case.

Acked-by: Ard Biesheuvel <ardb@kernel.org>
Acked-by: Jason A. Donenfeld <Jason@zx2c4.com>
Link: https://patch.msgid.link/20261003162827.155527-1-ebiggers@kernel.org
Signed-off-by: Eric Biggers <ebiggers@kernel.org>

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