There is a regular need in the kernel to provide a way to declare having a
dynamically sized set of trailing elements in a structure. Kernel code
should always use "flexible array members"[1] for these cases. The older
style of one-element or zero-length arrays should no longer be used[2].

Refactor the code a bit according to the use of a flexible-array member in
struct pqi_event_config instead of a one-element array, and use the
struct_size() helper.

This helps with the ongoing efforts to globally enable -Warray-bounds and
get us closer to being able to tighten the FORTIFY_SOURCE routines on
memcpy().

This issue was found with the help of Coccinelle and audited and fixed,
manually.

[1] https://en.wikipedia.org/wiki/Flexible_array_member
[2] https://www.kernel.org/doc/html/v5.10/process/deprecated.html#zero-length-and-one-element-arrays

Link: https://github.com/KSPP/linux/issues/79
Link: https://github.com/KSPP/linux/issues/109
Link: https://lore.kernel.org/r/20210810210741.GA58765@embeddedorSigned-off-by: NGustavo A. R. Silva <gustavoars@kernel.org>
Signed-off-by: NMartin K. Petersen <martin.petersen@oracle.com>