pgsql: Fix incorrect KeeperBlock macro in bump.c

Fix incorrect KeeperBlock macro in bump.c

The macro was missing a MAXALIGN around the sizeof(BumpContext) which
would cause problems detecting the keeper block on 32-bit systems that
have a MAXALIGN value of 8.

Thank you to Andres Freund, Tomas Vondra and Tom Lane for investigating
and testing.

Reported-by: Melanie Plageman, Tomas Vondra
Discussion: https://postgr.es/m/CAAKRu_Y6dZjiJEZghgNZp0Gjar1JVq-CH7XGDqExDVHnPgDjuw@mail.gmail.com
Discussion: https://postgr.es/m/a4a10b89-6ba8-4abd-b449-019aafff04fc@enterprisedb.com

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/705ec0565371033082e7b1c278afcb42aa5a7421

Modified Files
--------------
src/backend/utils/mmgr/bump.c | 5 +++--
1 file changed, 3 insertions(+), 2 deletions(-)