spg_mask() didn't take into account that pd_lower equal to SizeOfPageHeaderData
is still valid value.  This commit fixes that.  Backpatch to 11, where
spg_mask() pg_lower check was introduced.

Reported-by: Michael Paquier
Discussion: https://postgr.es/m/20200615131405.GM52676%40paquier.xyz
Backpatch-through: 11