VMCI: Annotate struct vmci_handle_arr with __counted_by

Prepare for the coming implementation by GCC and Clang of the __counted_by
attribute. Flexible array members annotated with __counted_by can have
their accesses bounds-checked at run-time checking via CONFIG_UBSAN_BOUNDS
(for array indexing) and CONFIG_FORTIFY_SOURCE (for strcpy/memcpy-family
functions).

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Reviewed-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/56bef519d982218176b59bbba64a3a308d8733d5.1696689091.git.christophe.jaillet@wanadoo.fr


Signed-off-by: Kees Cook <keescook@chromium.org>