fs: hpfs: Avoid multiple -Wflex-array-member-not-at-end warnings

fs: hpfs: Avoid multiple -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.

So, in order to avoid ending up with a flexible-array member in the
middle of other structs, we use the `struct_group_tagged()` helper
to create a new tagged `struct bplus_header_fixed`. This structure
groups together all the members of the flexible `struct bplus_header`
except the flexible array.

As a result, the array is effectively separated from the rest of the
members without modifying the memory layout of the flexible structure.
We then change the type of the middle struct member currently causing
trouble from `struct bplus_header` to `struct bplus_header_fixed`.

We also want to ensure that when new members need to be added to the
flexible structure, they are always included within the newly created
tagged struct. For this, we use `static_assert()`. This ensures that the
memory layout for both the flexible structure and the new tagged struct
is the same after any changes.

This approach avoids having to implement `struct bplus_header_fixed`
as a completely separate structure, thus preventing having to maintain
two independent but basically identical structures, closing the door
to potential bugs in the future.

We also use `container_of()` (via a wrapper) whenever we need to retrieve
a pointer to the flexible structure, through which we can access the
flexible-array member, if necessary.

So, with these changes, fix 26 of the following type of warnings:
fs/hpfs/hpfs.h:456:23: warning: structure containing a flexible array member is not at the end of another structure [-Wflex-array-member-not-at-end]
fs/hpfs/hpfs.h:498:23: 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: Mikulas Patocka <mpatocka@redhat.com>