ARM: mm: Allow an empty PMD in alloc_init_pte()

The Android patch titled "ARM: allow the kernel text section to
be made read-only" modifies alloc_init_pte() and adds a BUG_ON
to detect the case where a section mapping is being overwritten.
However the test doesn't allow for the legitimate case where the
PMD is empty, as can happen for kernels built with CONFIG_ARM_LPAE.
So extend the test to allow this.

Change-Id: I28eeaefd856bae63a5532980e41e0fd4d8922e79
Signed-off-by: Jon Medhurst <tixy@linaro.org>
Signed-off-by: John Stultz <john.stultz@linaro.org>