diag: Change to GFP_KERNEL in diagfwd_buffers_init()

In diagfwd_buffers_init() change the memory allocation to
from GFP_ATOMIC to GFP_KERNEL.
This patch also replaces spinlock with mutex to enable device
to go into sleep mode.

Change-Id: I8ea299a4287401a0a01ff7bbdd86c37ccd138480
Signed-off-by: Sreelakshmi Gownipalli <sgownipa@codeaurora.org>