arm: mm: fix pte allocation with CONFIG_FORCE_PAGES feature

CONFIG_FORCE_PAGES introduces a debug option to mark free
pages as read only in order to trigger a fault if any code
attempts to write to a page on the buddy list.

In order to achieve this it splits all section mappings in
to PAGE_SIZE pages during boot. While doing this split, it
wrongly allocates a pte (of page size) for each pmd. Linux
however in armv7 short descriptor format, uses same second
level pte for 2 consecutive pmds offset by the actual second
level table size. Refer comments in kernel file
arch/arm/include/asm/pgtable-2level.h for details.

Fix this by allocating pte for a section mapped pmd while
providing required offset if pte already exists.

Change-Id: Iadbaa5e71e4b220208a7275bf039a2a413349e42
Signed-off-by: Shiraz Hashim <shashim@codeaurora.org>