LU-11625 ofd: handle upgraded filter_fid properly

LU-11625 ofd: handle upgraded filter_fid properly

Since there have been several iterations of struct filter_fid stored
on disk, the current code wasn't checking for all of the possible
cases when trying to decide what action to take when accessing and
upgrading the xattr for new capabilities.

Properly check for the various different struct filter_fid sizes and
handle them appropriately.  Add a more verbose description of the
various cases so that this is more clear to others in the future.

Add decoding of filter_fid fields added for FLR in 2.11.

We should already be testing for upgrading the filter_fid xattr
from different OST versions in conf-sanity test_32d.

Signed-off-by: Andreas Dilger <adilger@whamcloud.com>
Change-Id: Ifef2292296236cb06ff7e8cd50caff4b133ebbe5