smb: use crypto_shash_digest() in symlink_hash()

Simplify symlink_hash() by using crypto_shash_digest() instead of an
init+update+final sequence.  This should also improve performance.

Signed-off-by: Eric Biggers <ebiggers@google.com>
Reviewed-by: Paulo Alcantara (SUSE) <pc@manguebit.com>
Signed-off-by: Steve French <stfrench@microsoft.com>