um: uml_dup_mmap() relies on ->mmap_sem being held, but activate_mm() doesn't hold it

... while calling uml_dup_mmap()

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
diff --git a/arch/um/kernel/skas/mmu.c b/arch/um/kernel/skas/mmu.c
index db2ef42..4947b31 100644
--- a/arch/um/kernel/skas/mmu.c
+++ b/arch/um/kernel/skas/mmu.c
@@ -101,7 +101,7 @@
 	return ret;
 }
 
-void arch_dup_mmap(struct mm_struct *oldmm, struct mm_struct *mm)
+void uml_setup_stubs(struct mm_struct *mm)
 {
 	struct page **pages;
 	int err, ret;