ext4: missing unlock in ext4_clear_request_list()

If the the li_request_list was empty then it returned with the lock
held.  Instead of adding a "goto unlock" I just removed that special
case and let it go past the empty list_for_each_safe().

Signed-off-by: Dan Carpenter <error27@gmail.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>