pgsql: Rename global variable backing DSA area

Rename global variable backing DSA area

The global variable backing the DSA area for Memory Context stats
reporting had a too generic name, rename to be more descriptive.
Independently reported by Peter and Laurenz.

Author: Daniel Gustafsson <daniel@yesql.se>
Reported-by: Peter Eisentraut <peter@eisentraut.org>
Reported-by: Laurenz Albe <laurenz.albe@cybertec.at>
Discussion: https://postgr.es/m/d51172bd4e7f4b07a18a0288ca1b1c28a71a5f6a.camel@cybertec.at
Discussion: https://postgr.es/m/25095db5-b595-4b85-9100-d358907c25b5@eisentraut.org

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/55ef7abf88c07e9f716846e645e1628c667c54a7

Modified Files
--------------
src/backend/utils/adt/mcxtfuncs.c | 14 +++++++-------
src/backend/utils/mmgr/mcxt.c     | 40 +++++++++++++++++++--------------------
src/include/utils/memutils.h      |  2 +-
3 files changed, 28 insertions(+), 28 deletions(-)