Avoid leaking FDs after an fsync failure.

Avoid leaking FDs after an fsync failure.

Fixes errors introduced in commit bc34223bc, as detected by Coverity.

In passing, report ENOSPC for a short write while padding a new wal file in
open_walfile, make certain that close_walfile closes walfile in all cases,
and improve a couple of comments.

Michael Paquier and Tom Lane