ion: fix system secure force alloc

Even when ION_FLAG_POOL_FORCE_ALLOC is set for secure alloc,
alloc_from_pool_preferred tries to get the pages from pool
first. In case if it enters split_page_from_secure_pool,
free_buffer_page can end up calling __free_pages on the page
from pool which is hyp assigned, resulting in issues for e.g.
a fault if page poisoning is enabled.

Change-Id: I68759dc17551a5705693506a5c137977d429fe36
Signed-off-by: Vinayak Menon <vinmenon@codeaurora.org>