ext4: drop lock_super()/unlock_super()

We don't need lock_super()/unlock_super() any more, since the places
where it is used, we are protected by the s_umount r/w semaphore.

Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
Cc: Marco Stornelli <marco.stornelli@gmail.com>