If the fatchain() call in chainalloc() returned an error, revert

marking the cluster run as in-use.

Tested by:	pho
Sponsored by:	The FreeBSD Foundation
MFC after:	2 weeks

sys/fs/msdosfs/msdosfs_fat.c

@@ -691,8 +691,11 @@ chainalloc(struct msdosfsmount *pmp, u_long start, u_long count,
 		pmp->pm_nxtfree = CLUST_FIRST;
 	pmp->pm_flags |= MSDOSFS_FSIMOD;
 	error = fatchain(pmp, start, count, fillwith);
-	if (error != 0)
+	if (error != 0) {
+		for (cl = start, n = count; n-- > 0;)
+			usemap_free(pmp, cl++);
 		return (error);
+	}
 #ifdef MSDOSFS_DEBUG
 	printf("clusteralloc(): allocated cluster chain at %lu (%lu clusters)\n",
 	    start, count);