fs/select: Annotate struct poll_list with __counted_by()

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

Signed-off-by: Thorsten Blum <thorsten.blum@toblux.com>
Link: https://lore.kernel.org/r/20240808150023.72578-2-thorsten.blum@toblux.com


Reviewed-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Signed-off-by: Christian Brauner <brauner@kernel.org>