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

src/common/file_utils.c

@@ -273,6 +273,7 @@ fsync_fname(const char *fname, bool isdir, const char *progname)
 	{
 		fprintf(stderr, _("%s: could not fsync file \"%s\": %s\n"),
 				progname, fname, strerror(errno));
+		(void) close(fd);
 		return -1;
 	}