crypto: md5 - Implement export_core() and import_core()

Since commit 9d7a0ab1 ("crypto: ahash - Handle partial blocks in
API"), the recently-added export_core() and import_core() methods in
struct shash_alg have effectively become mandatory (even though it is
not tested or enforced), since legacy drivers that need a fallback
depend on them.  Make crypto/md5.c compatible with these legacy drivers
by adding export_core() and import_core() methods to it.

Fixes: ba8ee22a ("crypto: md5 - Wrap library and add HMAC support")
Link: https://lore.kernel.org/r/20250906215417.89584-1-ebiggers@kernel.org


Signed-off-by: Eric Biggers <ebiggers@kernel.org>