nfsd: Don't force CRYPTO_LIB_SHA256 to be built-in

Now that nfsd is accessing SHA-256 via the library API instead of via
crypto_shash, there is a direct symbol dependency on the SHA-256 code
and there is no benefit to be gained from forcing it to be built-in.
Therefore, select CRYPTO_LIB_SHA256 from NFSD (conditional on NFSD_V4)
instead of from NFSD_V4, so that it can be 'm' if NFSD is 'm'.

Signed-off-by: Eric Biggers <ebiggers@kernel.org>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>