pgsql: Change GetMultiXactInfo() to return the next multixact offset

Change GetMultiXactInfo() to return the next multixact offset

This routine returned a number of members as a MultiXactOffset,
calculated based on the difference between the next-to-be-assigned
offset and the oldest offset.  However, this number is not actually an
offset but a number.

This type confusion comes from the original implementation of
MultiXactMemberFreezeThreshold(), in 53bb309d2d5a.  The number of
members is now defined as a uint64, large enough for MultiXactOffset.
This change will be used in a follow-up patch.

Reviewed-by: Naga Appani <nagnrik@gmail.com>
Discussion: https://postgr.es/m/aUyTvZMq2CLgNEB4@paquier.xyz

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/9cf746a453c15ffdf652a6d50683bfd82e654950

Modified Files
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src/backend/access/transam/multixact.c | 16 ++++++++--------
src/include/access/multixact.h         |  2 +-
2 files changed, 9 insertions(+), 9 deletions(-)