pgsql: Convert macros to static inline functions (bufpage.h)

Convert macros to static inline functions (bufpage.h)

Remove PageIsValid() and PageSizeIsValid(), which weren't used and
seem unnecessary.

Some code using these formerly-macros needs some adjustments because
it was previously playing loose with the Page vs. PageHeader types,
which is no longer possible with the functions instead of macros.

Reviewed-by: Amul Sul <sulamul(at)gmail(dot)com>
Discussion: https://www.postgresql.org/message-id/flat/5b558da8-99fb-0a99-83dd-f72f05388517%40enterprisedb.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/2cd2569c72b8920048e35c31c9be30a6170e1410

Modified Files
--------------
contrib/pageinspect/rawpage.c | 24 ++--
src/backend/storage/page/bufpage.c | 18 +--
src/bin/pg_checksums/pg_checksums.c | 2 +-
src/include/storage/bufpage.h | 257 ++++++++++++++++++++++--------------
src/include/storage/checksum_impl.h | 2 +-
5 files changed, 182 insertions(+), 121 deletions(-)