use the old relfilenode in the new tablespace. There might be another relation
in the new tablespace with the same relfilenode, so we must generate a fresh
relfilenode in the new tablespace.

The 8.3 patch to let deleted relation files linger as zero-length files until
the next checkpoint made this more obvious: moving a relation from one table
space another, and then back again, caused a collision with the lingering
file.

Back-patch to 8.1. The issue is present in 8.0 as well, but it doesn't seem
worth fixing there, because we didn't have protection from OID collisions
after OID wraparound before 8.1.

Report by Guillaume Lelarge.