pgsql: Rework locking code in GetMultiXactIdMembers

Rework locking code in GetMultiXactIdMembers

After commit 53c2a97a9266, the code flow around the "retry" goto label
in GetMultiXactIdMembers was confused about what was possible: we never
return there with a held lock, so there's no point in testing for one.
This realization lets us simplify the code a bit. While at it, make the
scope of a couple of local variables in the same function a bit tighter.

Per Coverity.

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/a0b808baef39e9f9465b7f63f2d735f35852aa21

Modified Files
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src/backend/access/transam/multixact.c | 53 ++++++++++++++--------------------
1 file changed, 22 insertions(+), 31 deletions(-)