PATCH: better error return for exFAT filesystem

[johnc]

Perform an explicit check for the unsupported exFAT MSDOS filesystem
instead of letting it fail mysteriously when it gets cluster sizes
of 0 from the normal fields.

This causes mount_msdos to report:
mount_msdos: /dev/sd1i on /root/mnt: filesystem not supported by kernel

Instead of the more obscure:
mount_msdos: /dev/sd1i on /root/mnt: Inapropriate file type or format

Index: msdosfs_vfsops.c
===================================================================
RCS file: /cvs/src/sys/msdosfs/msdosfs_vfsops.c,v
retrieving revision 1.93
diff -u -p -r1.93 msdosfs_vfsops.c
--- msdosfs_vfsops.c    24 Jan 2020 03:49:34 -0000      1.93
+++ msdosfs_vfsops.c    7 Aug 2020 19:52:04 -0000
@@ -298,6 +298,12 @@ msdosfs_mountfs(struct vnode *devvp, str
        b50 = (struct byte_bpb50 *)bsp->bs50.bsBPB;
        b710 = (struct byte_bpb710 *)bsp->bs710.bsBPB;
 
+       /* No support for exFAT filesystems */
+       if (!strncmp("EXFAT", bsp->bs33.bsOemName, 5)) {
+               error = EOPNOTSUPP;
+               goto error_exit;
+       }
+
        pmp = malloc(sizeof *pmp, M_MSDOSFSMNT, M_WAITOK | M_ZERO);
        pmp->pm_mountp = mp;