FUSE: Prevent trivial panic

When open(2) was invoked against a FUSE filesystem with an unexpected flags
value (no O_RDONLY / O_RDWR / O_WRONLY), an assertion fired, causing panic.

For now, prevent the panic by rejecting such VOP_OPENs with EINVAL.

This is not considered the correct long term fix, but does prevent an
unprivileged denial-of-service.

PR:		236329
Reported by:	asomers
Reviewed by:	asomers
Sponsored by:	Dell EMC Isilon

sys/fs/fuse/fuse_vnops.c

@@ -1174,6 +1174,9 @@ fuse_vnop_open(struct vop_open_args *ap)
 	if (fuse_isdeadfs(vp)) {
 		return ENXIO;
 	}
+	if ((mode & (FREAD | FWRITE)) == 0)
+		return EINVAL;
+
 	fvdat = VTOFUD(vp);
 
 	if (vnode_isdir(vp)) {