nfsd: don't provide pre/post-op attrs if fh_getattr fails

nfsd calls fh_getattr to get the latest inode attrs for pre/post-op
info. In the event that fh_getattr fails, it resorts to scraping cached
values out of the inode directly.

Since these attributes are optional, we can just skip providing them
altogether when this happens.

Signed-off-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Reviewed-by: Neil Brown <neilb@suse.de>