Staging: android: ion: fix to set cached element of pool always

Staging: android: ion: fix to set cached element of pool always

The allocated memory with kmalloc() can have invalid value. To
avoid using the invalid value, always set cached element.

This removes below UBSAN warning:
   UBSAN: Undefined behaviour in drivers/staging/android/ion/ion_page_pool.c:33:11
   load of value 152 is not a valid value for type '_Bool'
   ...
   [<ffffff9009b89e48>] __ubsan_handle_load_invalid_value+0x80/0x90
   [<ffffff900a2c4044>] ion_page_pool_alloc+0x154/0x180
   [<ffffff900a2c5b60>] ion_system_heap_allocate+0x2b8/0xa68
   [<ffffff900a2c0688>] ion_alloc+0x238/0x9c8
   ...

Change-Id: I86e0ee70404bb074dad3b73dccec31ebcf2c7c72
Signed-off-by: Seung-Woo Kim <sw0312.kim@samsung.com>