lib/crypto: md5: Add MD5 and HMAC-MD5 library functions

lib/crypto: md5: Add MD5 and HMAC-MD5 library functions

Add library functions for MD5, including HMAC support.  The MD5
implementation is derived from crypto/md5.c.  This closely mirrors the
corresponding SHA-1 and SHA-2 changes.

Like SHA-1 and SHA-2, support for architecture-optimized MD5
implementations is included.  I originally proposed dropping those, but
unfortunately there is an AF_ALG user of the PowerPC MD5 code
(https://lore.kernel.org/r/c4191597-341d-4fd7-bc3d-13daf7666c41@csgroup.eu/),
and dropping that code would be viewed as a performance regression.  We
don't add new software algorithm implementations purely for AF_ALG, as
escalating to kernel mode merely to do calculations that could be done
in userspace is inefficient and is completely the wrong design.  But
since this one already existed, it gets grandfathered in for now.  An
objection was also raised to dropping the SPARC64 MD5 code because it
utilizes the CPU's direct support for MD5, although it remains unclear
that anyone is using that.  Regardless, we'll keep these around for now.

Note that while MD5 is a legacy algorithm that is vulnerable to
practical collision attacks, it still has various in-kernel users that
implement legacy protocols.  Switching to a simple library API, which is
the way the code should have been organized originally, will greatly
simplify their code.  For example:

    MD5:
        drivers/md/dm-crypt.c (for lmk IV generation)
        fs/nfsd/nfs4recover.c
        fs/ecryptfs/
        fs/smb/client/
        net/{ipv4,ipv6}/ (for TCP-MD5 signatures)

    HMAC-MD5:
        fs/smb/client/
        fs/smb/server/

(Also net/sctp/ if it continues using HMAC-MD5 for cookie generation.
However, that use case has the flexibility to upgrade to a more modern
algorithm, which I'll be proposing instead.)

As usual, the "md5" and "hmac(md5)" crypto_shash algorithms will also be
reimplemented on top of these library functions.  For "hmac(md5)" this
will provide a faster, more streamlined implementation.

Link: https://lore.kernel.org/r/20250805222855.10362-2-ebiggers@kernel.org
Signed-off-by: Eric Biggers <ebiggers@kernel.org>