ext4: fix code documentatioon

Add a TODO to remember fixing REQ_FUA | REQ_PREFLUSH for fast commit
buffers. Also, fix a typo in top level comment in fast_commit.c

Signed-off-by: Harshad Shirwadkar <harshadshirwadkar@gmail.com>
Link: https://lore.kernel.org/r/20201106035911.1942128-15-harshadshirwadkar@gmail.com


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