xenfs/xensyms: respect hypervisor's "next" indication

The interface specifies the symnum field as an input and output; the
hypervisor sets it to the next sequential symbol's index. xensyms_next()
incrementing the position explicitly (and xensyms_next_sym()
decrementing it to "rewind") is only correct as long as the sequence of
symbol indexes is non-sparse. Use the hypervisor-supplied value instead
to update the position in xensyms_next(), and use the saved incoming
index in xensyms_next_sym().

Cc: [e-mail]
Fixes: a11f4f0a4e18 ("xen: xensyms support")
Signed-off-by: Jan Beulich <[e-mail]>
Reviewed-by: Juergen Gross <[e-mail]>
Message-ID: <[e-mail]>
Signed-off-by: Juergen Gross <[e-mail]>

drivers/xen/xenfs/xensyms.c

···
 			return -ENOMEM;
 
 		set_xen_guest_handle(symdata->name, xs->name);
-		symdata->symnum--; /* Rewind */
+		symdata->symnum = symnum; /* Rewind */
 
 		ret = HYPERVISOR_platform_op(&xs->op);
 		if (ret < 0)
···
 {
 	struct xensyms *xs = m->private;
 
-	xs->op.u.symdata.symnum = ++(*pos);
+	*pos = xs->op.u.symdata.symnum;
 
 	if (xensyms_next_sym(xs))
 		return NULL;