Correct handling of fsync failures with tar mode of walmethods.c

Correct handling of fsync failures with tar mode of walmethods.c

This file has been missing the fact that it needs to report back to
callers a proper failure on fsync calls.  I have spotted the one in
tar_finish() while Kuntal has spotted the one in tar_close().

Backpatch down to 10 where this code has been introduced.

Reported by: Michael Paquier, Kuntal Ghosh
Author: Michael Paquier
Reviewed-by: Kuntal Ghosh, Magnus Hagander
Discussion: https://postgr.es/m/20180625024356.GD1146@paquier.xyz