nfsd: v4 support requires CRYPTO_SHA256

The new nfsdcld client tracking operations use sha256 to compute hashes
of the kerberos principals, so make sure CRYPTO_SHA256 is enabled.

Fixes: 6ee95d1c

 ("nfsd: add support for upcall version 2")
Reported-by: Jamie Heilman <jamie@audible.transient.net>
Signed-off-by: Scott Mayhew <smayhew@redhat.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>