reiserfs: Initialize inode keys properly

reiserfs_read_locked_inode() didn't initialize key length properly. Use
_make_cpu_key() macro for key initialization so that all key member are
properly initialized.

CC: stable@vger.kernel.org
Reported-by:  <syzbot+d94d02749498bb7bab4b@syzkaller.appspotmail.com>
Signed-off-by: Jan Kara <jack@suse.cz>