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.

---
 arch/i386/xen/mmu.c |    5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

===================================================================

@@ -558,6 +558,9 @@ void xen_exit_mmap(struct mm_struct *mm)
 	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);
 }