Don't use elog() in src/port/pwrite.c.

Nothing broke because of this oversight yet, but it would fail to link
if we tried to use pg_pwrite() in frontend code on a system that lacks
pwrite().  Use an assertion instead.  Also pgindent while here.

Discussion: https://postgr.es/m/CA%2BhUKGL57RvoQsS35TVPnQoPYqbtBixsdRhynB8NpcUKpHTTtg%40mail.gmail.com

src/port/pwrite.c

@@ -137,14 +137,14 @@ pg_pwritev_with_retry(int fd, const struct iovec *iov, int iovcnt, off_t offset)
 		/* Are they all done? */
 		if (iovcnt == 0)
 		{
-			if (part > 0)
-				elog(ERROR, "unexpectedly wrote more than requested");
+			/* We don't expect the kernel to write more than requested. */
+			Assert(part == 0);
 			break;
 		}
 
 		/*
-		 * Move whatever's left to the front of our mutable copy and adjust the
-		 * leading iovec.
+		 * Move whatever's left to the front of our mutable copy and adjust
+		 * the leading iovec.
 		 */
 		Assert(iovcnt > 0);
 		memmove(iov_copy, iov, sizeof(*iov) * iovcnt);