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@gmail.com>
Discussion: https://www.postgresql.org/message-id/flat/5b558da8-99fb-0a99-83dd-f72f05388517%40enterprisedb.com