dm: Annotate struct dm_bio_prison 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).

As found with Coccinelle[1], add __counted_by for struct dm_bio_prison.

[1] https://github.com/kees/kernel-tools/blob/trunk/coccinelle/examples/counted_by.cocci

Cc: Alasdair Kergon <[e-mail]>
Cc: Mike Snitzer <[e-mail]>
Cc: [e-mail]
Reviewed-by: "Gustavo A. R. Silva" <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Kees Cook <[e-mail]>

drivers/md/dm-bio-prison-v1.c

···
 struct dm_bio_prison {
 	mempool_t cell_pool;
 	unsigned int num_locks;
-	struct prison_region regions[];
+	struct prison_region regions[] __counted_by(num_locks);
 };
 
 static struct kmem_cache *_cell_cache;