anthias:17614185:Bluetooth: Getbuf returning invalid buffer pointer

b/17623865

Getbuff is checking for the pool which can accommodate
requested size, but it changing the pool with a restricted
check and not checking for the free memory size in that pool.

Added a check to avoid return the wrong pool free buf pointer
and avoid gki get_buffer overflow.

Change-Id: I142870c989fc37f49c2d9b411d5eddbfba59bd9d