ext4: Change the wrong param comment for ext4_trim_all_free

at ext4_trim_all_free() comment, there is no longer an @e4b parameter,
instead it is @group.

Reported-by: Andreas Dilger <adilger@dilger.ca>
Signed-off-by: Tao Ma <boyu.mt@taobao.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>