[Server] Use length returned by read and fix an alignment check in pgRead

[smithdh]

Changes XrdOssDF and XrdOucCacheIO pgRead to use the length returned by read when calculating the checksums, rather than the requested read size. Believed to fix issue #1507 (due to XrdOucCacheIO pgRead).

Also removes an alignment check which may have been inadvertently re-added to XrdOucCacheIO::pgRead during a rebase.

[abh3]

Thank you David for catching this. An obvious and stupid mistake. That said, I did ask about the removal the the test for pgRead() as it seem at least the offset check should stay.

[smithdh]

(some discussion going on by email with Andy & Matevz about the alignment check(s) in XrdOucCacheIO::pgRead)

[abh3]

You are absolutely correct. Somewhere along the way we started using the CacheIO object for all IO requests, cache or not. Since pgRead and pgWrite were extended to be able to do unaligned reads and writes the change was indeed necessary.