ceph: fix SEEK_CUR, SEEK_SET regression

Commit 06222e491e663dac939f04b125c9dc52126a75c4 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: Roel Kluin <roel.kluin@gmail.com>
Signed-off-by: Sage Weil <sage@newdream.net>