load_chunk_table(): Don't trust afhi->chunks_total.

load_chunk_table(): Don't trust afhi->chunks_total.

Both callers pass a buffer to load_chunk_table() to initialize
afhi->chunk_table. The function does not check the size of the passed
buffer but relies on afhi->chunks_total to tell how many values to
copy from the buffer.

In other words, the buffer size gives an upper bound for the number of
chunks in the chunk table, but this bould is not checked. This patch
makes the code more robust by adding a suitable check. To this aim,
load_chunk_table() is changed to receive a pointer to an osl object
(which contains the buffer size) rather than only the buffer.

One caller, load_afd(), initializes the chunk table from a shared
memory area buffer. It does not know or care about the buffer size
so far. We introduce the the new IPC helper shm_size() to let it tell
the size of the area and pass it to load_chunk_table().