Fix write(2) returns zero bug from 933ec99

For generic_write_checks with 2 args, we can exit when it returns zero because
it means count is zero. However this is not the case for generic_write_checks
with 4 args, where zero means no error.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Haakan T Johansson <f96hajo@chalmers.se>
Signed-off-by: Chunwei Chen <david.chen@osnexus.com>
Closes openzfs#5720 
Closes openzfs#5726

module/zfs/zpl_file.c

@@ -387,17 +387,19 @@ zpl_iter_write(struct kiocb *kiocb, struct iov_iter *from)
 
 	count = iov_iter_count(from);
 	ret = generic_write_checks(file, &kiocb->ki_pos, &count, isblk);
+	if (ret)
+		return (ret);
 #else
 	/*
 	 * XXX - ideally this check should be in the same lock region with
 	 * write operations, so that there's no TOCTTOU race when doing
 	 * append and someone else grow the file.
 	 */
 	ret = generic_write_checks(kiocb, from);
-	count = ret;
-#endif
 	if (ret <= 0)
 		return (ret);
+	count = ret;
+#endif
 
 	if (from->type & ITER_KVEC)
 		seg = UIO_SYSSPACE;