Fix the following gcc warning:

fs/ext4/super.c:599:27: warning: variable 'es' set but not used [-Wunused-but-set-variable]
  struct ext4_super_block *es;
                           ^~
Fixes: 2ea2fc775321 ("ext4: save all error info in save_error_info() and drop ext4_set_errno()")
Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Jason Yan <yanaijie@huawei.com>
Link: https://lore.kernel.org/r/20200402033939.25303-1-yanaijie@huawei.comSigned-off-by: Theodore Ts'o <tytso@mit.edu>