ecryptfs: bound session_key_encryption_key_bytes in auth tok

Let ecryptfs_verify_auth_tok_from_key() validate the password's
session_key_encryption_key_bytes length. When it is larger than
ECRYPTFS_MAX_KEY_BYTES, md5() reads past session_key_encryption_key[].
Reject such auth toks.

Fixes: 9c79f34f7ee7 ("eCryptfs: Filename Encryption: Tag 70 packets")
Cc: stable@vger.kernel.org
Reported-by: syzbot+03c68e42ce4b33f7ce9b@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=03c68e42ce4b33f7ce9b
Tested-by: syzbot+03c68e42ce4b33f7ce9b@syzkaller.appspotmail.com
Signed-off-by: Adriano Cordova <adrianox@gmail.com>
Signed-off-by: Tyler Hicks <code@tyhicks.com>