fs/ntfs3: Fix null-ptr-deref on inode->i_op in ntfs_lookup()

In the Linux kernel, the following vulnerability has been resolved:

fs/ntfs3: Fix null-ptr-deref on inode->i_op in ntfs_lookup()

Syzbot reported a null-ptr-deref bug:

ntfs3: loop0: Different NTFS' sector size (1024) and media sector size
(512)
ntfs3: loop0: Mark volume as dirty due to NTFS errors
general protection fault, probably for non-canonical address
0xdffffc0000000001: 0000 [#1] PREEMPT SMP KASAN
KASAN: null-ptr-deref in range [0x0000000000000008-0x000000000000000f]
RIP: 0010:d_flags_for_inode fs/dcache.c:1980 [inline]
RIP: 0010:__d_add+0x5ce/0x800 fs/dcache.c:2796
Call Trace:
<TASK>
d_splice_alias+0x122/0x3b0 fs/dcache.c:3191
lookup_open fs/namei.c:3391 [inline]
open_last_lookups fs/namei.c:3481 [inline]
path_openat+0x10e6/0x2df0 fs/namei.c:3688
do_filp_open+0x264/0x4f0 fs/namei.c:3718
do_sys_openat2+0x124/0x4e0 fs/open.c:1310
do_sys_open fs/open.c:1326 [inline]
__do_sys_open fs/open.c:1334 [inline]
__se_sys_open fs/open.c:1330 [inline]
__x64_sys_open+0x221/0x270 fs/open.c:1330
do_syscall_x64 arch/x86/entry/common.c:50 [inline]
do_syscall_64+0x3d/0xb0 arch/x86/entry/common.c:80
entry_SYSCALL_64_after_hwframe+0x63/0xcd

If the MFT record of ntfs inode is not a base record, inode->i_op can be
NULL. And a null-ptr-deref may happen:

ntfs_lookup()
dir_search_u() # inode->i_op is set to NULL
d_splice_alias()
__d_add()
d_flags_for_inode() # inode->i_op->get_link null-ptr-deref

Fix this by adding a Check on inode->i_op before calling the
d_splice_alias() function.