"inconsistent page found" with checksum and wal_consistency_checking enabled

Currently, page checksum is not masked by Page masking routines used
by wal_consistency_checking facility. So, when running `make installcheck`
with data checksum enabled and wal_consistency_checking='all', it easily
and consistently FATALs with "inconsistent page found".

If anything needs to be masked on Page to perform / pass wal consistency
checking, definitely checksum is not going to match and hence must be
masked as well. Attaching patch to fix the same, installcheck passes with
checksums enabled and wal_consistency_checking='all' with the fix.

Clubbed to perform the masking with lsn as it sounds logical to have them
together, as lsn is masked is all the cases so far and such is needed for
checksum as well.

Thank You,
Ashwin Agrawal