reiserfs: Initialize sec->length in reiserfs_security_init().

syzbot is reporting that sec->length is not initialized.

Since security_inode_init_security() returns 0 when initxattrs is provided
but call_int_hook(inode_init_security) returned -EOPNOTSUPP, control will
reach to "if (sec->length && ...) {" without initializing sec->length.

Reported-by: syzbot <syzbot+00a3779539a23cbee38c@syzkaller.appspotmail.com>
Closes: https://syzkaller.appspot.com/bug?extid=00a3779539a23cbee38c


Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Fixes: 52ca4b64 ("reiserfs: Switch to security_inode_init_security()")
Signed-off-by: Paul Moore <paul@paul-moore.com>