pgsql: Add MultiXactOffsetStorageSize() to multixact_internal.h

Add MultiXactOffsetStorageSize() to multixact_internal.h

This function calculates in bytes the storage taken between two
multixact offsets. This will be used in an upcoming patch, introduced
separately here as this piece can be useful on its own.

Author: Naga Appani <nagnrik@gmail.com>
Co-authored-by: Michael Paquier <michael@paquier.xyz>
Discussion: /messages/by-id/aUyTvZMq2CLgNEB4@paquier.xyz

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/0e3ad4b96aedee57fc2694e28486fe0ceca8110a

Modified Files
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src/include/access/multixact_internal.h | 10 ++++++++++
1 file changed, 10 insertions(+)