bcachefs: Avoid -Wflex-array-member-not-at-end warnings

bcachefs: Avoid -Wflex-array-member-not-at-end warnings

-Wflex-array-member-not-at-end was introduced in GCC-14, and we are
getting ready to enable it, globally.

Refactor a couple of structs that contain flexible arrays in the
middle by replacing them with unions.

So, with these changes, fix the following warnings:

fs/bcachefs/disk_accounting.c:429:51: warning: structure containing a flexible array member is not at the end of another structure [-Wflex-array-member-not-at-end]
fs/bcachefs/ec_types.h:8:41: warning: structure containing a flexible array member is not at the end of another structure [-Wflex-array-member-not-at-end]

Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>