In rev. 1.17 (r33548) of msdosfs_fat.c, relative cluster numbers were

replaced by file relative sector numbers as the buffer block number when
zero-padding a file during extension. Revert the change, it causes wrong
blocks filled with zeroes on seeking beyond end of file.

PR:	kern/47628
Submitted by:	tegge
MFC after:	3 days

sys/fs/msdosfs/msdosfs_fat.c

@@ -1065,13 +1065,13 @@ extendfile(dep, count, bpp, ncp, flags)
 					    pmp->pm_bpcluster, 0, 0, 0);
 				else {
 					bp = getblk(DETOV(dep),
-					    de_cn2bn(pmp, frcn++),
+					    frcn++,
 					    pmp->pm_bpcluster, 0, 0, 0);
 					/*
 					 * Do the bmap now, as in msdosfs_write
 					 */
 					if (pcbmap(dep,
-					    de_bn2cn(pmp, bp->b_lblkno),
+					    bp->b_lblkno,
 					    &blkno, 0, 0))
 						bp->b_blkno = -1;
 					if (bp->b_blkno == -1)