ext4: move the jbd2 wrapper functions out of super.c

Move the jbd2 wrapper functions which start and stop handles out of
super.c, where they don't really logically belong, and into
ext4_jbd2.c.

Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>

diff --cc fs/ext4/ext4.h
Simple merge

diff --cc fs/ext4/ext4_jbd2.c
Simple merge

diff --cc fs/ext4/super.c
Simple merge