fs/ntfs3: Fix missing i_op in ntfs_read_mft

There is null pointer dereference because i_op == NULL.
The bug happens because we don't initialize i_op for records in $Extend.
Fixes: 82cae269

 ("fs/ntfs3: Add initialization of super block")

Reported-by: Liangbin Lian <jjm2473@gmail.com>
Signed-off-by: Konstantin Komarov <almaz.alexandrovich@paragon-software.com>