xen: execve's error paths don't pin the mm before unpinning

execve's error paths don't activate (and therefore pin) the mm before
calling exit_mmap to free it up, so don't try to unpin unless it is
actually pinned. This prevents a BUG_ON from triggering.

Signed-off-by: Jeremy Fitzhardinge <[e-mail]>
Cc: Christian Ostheimer <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

arch/i386/xen/mmu.c

···
 	put_cpu();
 
 	spin_lock(&mm->page_table_lock);
-	xen_pgd_unpin(mm->pgd);
+
+	/* pgd may not be pinned in the error exit path of execve */
+	if (PagePinned(virt_to_page(mm->pgd)))
+		xen_pgd_unpin(mm->pgd);
 	spin_unlock(&mm->page_table_lock);
 }