Commit 06222e49 got the if wrong so that
it always evaluates as true.  This is semantically harmless, but makes
SEEK_CUR and SEEK_SET needlessly query the server.

Rewrite the if to explicitly enumerate the cases we DO need a valid i_size
to make this code less fragile.
Reported-by: NRoel Kluin <roel.kluin@gmail.com>
Signed-off-by: NSage Weil <sage@newdream.net>