ext4: fix wrong list_splice in ext4_fc_cleanup

ext4: fix wrong list_splice in ext4_fc_cleanup

After full/fast commit, entries in staging queue are promoted to main
queue. In ext4_fs_cleanup function, it splice to staging queue to
staging queue.

Fixes: aa75f4d3daaeb ("ext4: main fast-commit commit path")
Signed-off-by: Daejun Park <daejun7.park@samsung.com>
Reviewed-by: Harshad Shirwadkar <harshadshirwadkar@gmail.com>
Link: https://lore.kernel.org/r/20201230094851epcms2p6eeead8cc984379b37b2efd21af90fd1a@epcms2p6
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Cc: stable@kernel.org