ext4: remove set but not used variable 'es' in ext4_jbd2.c

Fix the following gcc warning:

fs/ext4/ext4_jbd2.c:341:30: 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/20200402034759.29957-1-yanaijie@huawei.com
Signed-off-by: Theodore Ts'o <tytso@mit.edu>