Change the test for num_fats from != 0 to a check for specifically 1.

Although it's theoretically possible that num_fats == 2 for a TexFAT volume (or
an implementation that doesn't do the full TexFAT but does support 2 FAT
tables), the rest of the code doesn't currently DTRT if it's 2 (in particular,
not handling the case of ActiveFat pointing at the second FAT area), so we'll
disallow that as well, as well as dealing with corrupted images that have a
trash non-zero value.

Signed-off-by: Valdis Kletnieks <valdis.kletnieks@vt.edu>

--- a/fs/exfat/super.c	2020-02-14 17:45:02.262274632 -0500
+++ b/fs/exfat/super.c	2020-02-14 17:46:37.200343723 -0500
@@ -450,7 +450,7 @@ static int __exfat_fill_super(struct sup
 	}

 	p_bpb = (struct pbr64 *)p_pbr;
-	if (!p_bpb->bsx.num_fats) {
+	if (p_bpb->bsx.num_fats  != 1) {
 		exfat_msg(sb, KERN_ERR, "bogus number of FAT structure");
 		ret = -EINVAL;
 		goto free_bh;