pgsql: Avoid unsatisfied-external-reference errors in static inlines.

Avoid unsatisfied-external-reference errors in static inlines.

Commit 9c727360b neglected the lesson we've learned before:
protect references to backend global variables with #ifndef FRONTEND.

Since there's already a place for static inlines in this file,
move the just-converted functions to that stanza.  Undo the
entirely gratuitous de-macroization of RelationGetNumberOfBlocks
(that one may be okay, since it has no global variable references,
but it's also pointless).

Per buildfarm.

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/27d2693187d1bcf2563ee7142ba37d4788c8d52b

Modified Files
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src/include/storage/bufmgr.h | 166 +++++++++++++++++++++----------------------
1 file changed, 80 insertions(+), 86 deletions(-)