common: Move MD5 hash to hash_algo[] array.

MD5 is being called directly in some places, but it is not available
via hash_lookup_algo("md5"). This is inconsistent with other hasing
routines. To resolve this, add an "md5" entry to hash_algos[].

The #ifdef clause looks funnier than those for other entries. This is
because both MD5 and SPL_MD5 configs exist, whereas the other hashes
do not have "SPL_" entries. The long term plan is to get rid of the
ifdefs, so those should not be expected to survive much longer.

The md5 entry does not have .hash_init/update/finish members. That's
okay because hash_progressive_lookup_algo() will catch that, and
return -EPROTONOSUPPORT, while hash_lookup_algo() will return the
correct pointer.

Signed-off-by: Alexandru Gagniuc <mr.nuke.me@gmail.com>
[trini: Use CONFIG_IS_ENABLED not IS_ENABLED for MD5 check]
Signed-off-by: Tom Rini <trini@konsulko.com>