ext4: add a mount opt to forcefully turn fast commits on

This is a debug only mount option that forcefully turns fast commits
on at mount time.

Signed-off-by: Harshad Shirwadkar <harshadshirwadkar@gmail.com>
Link: https://lore.kernel.org/r/20201015203802.3597742-9-harshadshirwadkar@gmail.com
Signed-off-by: Theodore Ts'o <tytso@mit.edu>