os/FileStore: use _fgetattrs() for _setattrs()

os/FileStore: use _fgetattrs() for _setattrs()

If we're using omap for spillover, we need to load the xattrs so that
they can be removed if necessary.

Note that we subtly change the failure behavior here to be more
strict: if we can't open the file, we fail, whereas before we did not.

Signed-off-by: Sage Weil <sage@inktank.com>