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

Fixes: 10d7c95 ("fs/ntfs3: add delayed-allocation (delalloc) support")
Reported-by: syzbot+7be88937363ac7ab7bb0@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=7be88937363ac7ab7bb0
Tested-by: syzbot+7be88937363ac7ab7bb0@syzkaller.appspotmail.com
Signed-off-by: Edward Adam Davis <eadavis@qq.com>
Signed-off-by: Konstantin Komarov <almaz.alexandrovich@paragon-software.com>

Author: ea1davis

fs/ntfs3/inode.c

@@ -827,6 +827,11 @@ static int ntfs_iomap_begin(struct inode *inode, loff_t offset, loff_t length,
 		return err;
 	}
 
+	if (!clen) {
+		/* broken file? */
+		return -EINVAL;
+	}
+
 	if (lcn == EOF_LCN) {
 		/* request out of file. */
 		if (flags & IOMAP_REPORT) {
@@ -860,11 +865,6 @@ static int ntfs_iomap_begin(struct inode *inode, loff_t offset, loff_t length,
 		return 0;
 	}
 
-	if (!clen) {
-		/* broken file? */
-		return -EINVAL;
-	}
-
 	iomap->bdev = inode->i_sb->s_bdev;
 	iomap->offset = offset;
 	iomap->length = ((loff_t)clen << cluster_bits) - off;