pgsql: Fix alignment problems with SharedInvalSmgrMsg.

Fix alignment problems with SharedInvalSmgrMsg.

SharedInvalSmgrMsg can't require 8-byte alignment, because then
SharedInvalidationMessage will require 8-byte alignment, which will
then cause ParseCommitRecord to fail on machines that are picky
about alignment, because it assumes that everything that gets
packed into a commit record requires only 4-byte alignment.

Another problem with 05d4cbf9b6ba708858984b01ca0fc56d59d4ec7c.

Discussion: http://postgr.es/m/3825454.1664310917@sss.pgh.pa.us

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/0222be137188ac3912d244d832a328be0944f3e9

Modified Files
--------------
src/backend/utils/cache/inval.c | 9 +++++++--
src/include/storage/sinval.h    | 7 +++++--
2 files changed, 12 insertions(+), 4 deletions(-)