ion: retry during race with shrinking/allocating

There exists a race between shrinking/allocating

Thread 1:                           Thread:2
mutex_lock(&sheap->alloc_lock)
ion_secure_cma_add_to_pool()
mutex_lock(&sheap->chunk_lock)
chunk->cnt = 0
mutex_unlock(&sheap->chunk_lock)
                                    ion_secure_cma_shrinker
                                    mutex_lock(&sheap->chunk_lock)
                                    __ion_secure_cma_shrink_pool
                                    chunk->cnt == 0 for the
                                       just allocated chunk
                                    ion_secure_cma-free_chunk
ion_secure_cma_alloc_from_pool
no memory to allocate!

If this race happens, just retry adding memory to the pool instead
of BUGing out the system.

Change-Id: Ic95782db6d88a33f01dcc5248fc957ca0f378078
Signed-off-by: Laura Abbott <lauraa@codeaurora.org>