powerpc/mm: Fix bug in gup_hugepd()

Commit a4fe3ce7 introduced a new
get_user_pages() path for hugepages on powerpc.  Unfortunately, there
is a bug in it's loop logic, which can cause it to overrun the end of
the intended region.  This came about by copying the logic from the
normal page path, which assumes the address and end parameters have
been pagesize aligned at the top-level.  Since they're not *hugepage*
size aligned, the simplistic logic could step over the end of the gup
region without triggering the loop end condition.

This patch fixes the bug by using the technique that the normal page
path uses in levels above the lowest to truncate the ending address to
something we know we'll match with.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>