ARM: 7438/1: fill possible PMD empty section gaps

cherry-picking from kernel v3.5

On ARM with the 2-level page table format, a PMD entry is represented by
two consecutive section entries covering 2MB of virtual space.

However, static mappings always were allowed to use separate 1MB section
entries.  This means in practice that a static mapping may create half
populated PMDs via create_mapping().

Since commit 0536bdf3 (ARM: move iotable mappings within the vmalloc
region) those static mappings are located in the vmalloc area. We must
ensure no such half populated PMDs are accessible once vmalloc() or
ioremap() start looking at the vmalloc area for nearby free virtual
address ranges, or various things leading to a kernel crash will happen.

Change-Id: I4936a18717d9fce5921f0399ef60cfdcd0ba6100
Signed-off-by: Nicolas Pitre <nico@linaro.org>
Reported-by: Santosh Shilimkar <santosh.shilimkar@ti.com>
Tested-by: "R, Sricharan" <r.sricharan@ti.com>
Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
Cc: stable@vger.kernel.org
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>