ocfs2: avoid -Wflex-array-member-not-at-end warning

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

Use the new TRAILING_OVERLAP() helper to fix the following warning:

fs/ocfs2/xattr.c:52:41: warning: structure containing a flexible array member is not at the end of another structure [-Wflex-array-member-not-at-end]

This helper creates a union between a flexible-array member (FAM) and a
set of MEMBERS that would otherwise follow it.

This overlays the trailing MEMBER struct ocfs2_extent_rec er; onto the FAM
struct ocfs2_xattr_value_root::xr_list.l_recs[], while keeping the FAM and
the start of MEMBER aligned.

The static_assert() ensures this alignment remains, and it's intentionally
placed inmediately after the related structure --no blank line in between.

Link: https://lkml.kernel.org/r/aRKm_7aN7Smc3J5L@kspp


Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Reviewed-by: Heming Zhao <heming.zhao@suse.com>
Acked-by: Joseph Qi <joseph.qi@linux.alibaba.com>
Cc: Mark Fasheh <mark@fasheh.com>
Cc: Joel Becker <jlbec@evilplan.org>
Cc: Junxiao Bi <junxiao.bi@oracle.com>
Cc: Changwei Ge <gechangwei@live.cn>
Cc: Jun Piao <piaojun@huawei.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>