From: Martin Schwidefsky <schwidefsky@de.ibm.com>

Fix endless loop in get_user_pages() on s390.  It happens only on s/390
because pte_dirty always returns 0.  For all other architectures this is an
optimization.

In the case of "write && !pte_dirty(pte)" follow_page() returns NULL.  On all
architectures except s390 handle_pte_fault() will then create a pte with
pte_dirty(pte)==1 because write_access==1.  In the following, second call to
follow_page() all is fine.  With the physical dirty bit patch pte_dirty() is
always 0 for s/390 because the dirty bit doesn't live in the pte.