ntfs: avoid calling post_write_mst_fixup() for invalid index_block

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

ntfs: avoid calling post_write_mst_fixup() for invalid index_block

ntfs_icx_ib_sync_write() calls post_write_mst_fixup() when ntfs_ib_write()
returns an error, intending to restore the buffer after a failed write.

However, ntfs_ib_write() returns an error immediately if
pre_write_mst_fixup() validation fails. The caller,
ntfs_icx_ib_sync_write(), interprets any error as a write failure
requiring rollback. It does not differentiate between I/O errors and
validation failures, and calls post_write_mst_fixup() anyway.

Since post_write_mst_fixup() assumes that the index_block contents is
correct, it doesn't perform the boundary checks, which results in
out-of-bounds memory access.

An attacker can craft a malicious NTFS image with:
- large index_block.usa_ofs offset, pointing outside the ntfs_record
- index_block.usa_count = 0, causing integer underflow
- or index_block.usa_count larger than actual number of sectors in the
ntfs_record, causing out-of-bounds access

KASAN reports describing the memory corruption:
==================================================================
BUG: KASAN: slab-out-of-bounds in post_write_mst_fixup+0x19c/0x1d0
Read of size 2 at addr ffff8881586c9018 by task p/9428
Call Trace:
<TASK>
dump_stack_lvl+0x100/0x190
print_report+0x139/0x4ad
? post_write_mst_fixup+0x19c/0x1d0
? __virt_addr_valid+0x262/0x500
? post_write_mst_fixup+0x19c/0x1d0
kasan_report+0xe4/0x1d0
? post_write_mst_fixup+0x19c/0x1d0
post_write_mst_fixup+0x19c/0x1d0
ntfs_icx_ib_sync_write+0x179/0x220
ntfs_inode_sync_filename+0x83d/0x1080
__ntfs_write_inode+0x1049/0x1480
ntfs_file_fsync+0x131/0x9b0
==================================================================
BUG: KASAN: slab-out-of-bounds in post_write_mst_fixup+0x1aa/0x1d0
Write of size 2 at addr ffff8881586c91fe by task p/9428
Call Trace:
<TASK>
dump_stack_lvl+0x100/0x190
print_report+0x139/0x4ad
? post_write_mst_fixup+0x1aa/0x1d0
? __virt_addr_valid+0x262/0x500
? post_write_mst_fixup+0x1aa/0x1d0
kasan_report+0xe4/0x1d0
? post_write_mst_fixup+0x1aa/0x1d0
post_write_mst_fixup+0x1aa/0x1d0
ntfs_icx_ib_sync_write+0x179/0x220
ntfs_inode_sync_filename+0x83d/0x1080
__ntfs_write_inode+0x1049/0x1480
ntfs_file_fsync+0x131/0x9b0
==================================================================

Let's move the post_write_mst_fixup() call to ntfs_ib_write().
The ntfs_ib_write() function calls pre_write_mst_fixup() at the beginning.
If the index_block contents is invalid, pre_write_mst_fixup() fails and
ntfs_ib_write() returns early without calling post_write_mst_fixup() on
bad index_block.