fs/ntfs3: prevent uninitialized lcn caused by zero len

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

fs/ntfs3: prevent uninitialized lcn caused by zero len

syzbot reported a uninit-value in ntfs_iomap_begin [1].

Since runs was not touched yet, run_lookup_entry() immediately fails and returns false, which makes the value of "*len" 0. Simultaneously, the new value and err value are also 0, causing the logic in attr_data_get_block_locked() to jump directly to ok, ultimately resulting in *lcn being triggered before it is set [1].

In ntfs_iomap_begin(), the check for a 0 value in clen is moved forward to before updating lcn to avoid this [1].

[1] BUG: KMSAN: uninit-value in ntfs_iomap_begin+0x8c0/0x1460 fs/ntfs3/inode.c:825 ntfs_iomap_begin+0x8c0/0x1460 fs/ntfs3/inode.c:825 iomap_iter+0x9b7/0x1540 fs/iomap/iter.c:110

Local variable lcn created at: ntfs_iomap_begin+0x15d/0x1460 fs/ntfs3/inode.c:786