Patch 0001 gets rid of the unconditional lseek() calls for SLRU I/O, as a small follow-up to commit c24dcd0c. Patch 0002 gets rid of a few places that usually do a good job of avoiding lseek() calls while reading and writing WAL, but it seems better to have no code at all.
I reviewed the changes and they look good to me. Code looks much cleaner after 2nd patch.
After these changes, only one usage of SLRU_SEEK_FAILED remains in SimpleLruDoesPhysicalPageExist().
