ksmbd: Annotate struct copychunk_ioctl_req with __counted_by_le()

Add the __counted_by_le compiler attribute to the flexible array member
Chunks to improve access bounds-checking via CONFIG_UBSAN_BOUNDS and
CONFIG_FORTIFY_SOURCE.

Change the data type of the flexible array member Chunks from __u8[] to
struct srv_copychunk[] for ChunkCount to match the number of elements in
the Chunks array. (With __u8[], each srv_copychunk would occupy 24 array
entries and the __counted_by compiler attribute wouldn't be applicable.)

Use struct_size() to calculate the size of the copychunk_ioctl_req.

Read Chunks[0] after checking that ChunkCount is not 0.

Signed-off-by: Thorsten Blum <thorsten.blum@linux.dev>
Acked-by: Namjae Jeon <linkinjeon@kernel.org>
Signed-off-by: Steve French <stfrench@microsoft.com>