ext4: dedpulicate the code to wait on inode that's being committed

This patch removes the deduplicates the code that implements waiting
on inode that's being committed. That code is moved into a new
function.

Suggested-by: Jan Kara <jack@suse.cz>
Signed-off-by: Harshad Shirwadkar <harshadshirwadkar@gmail.com>
Link: https://lore.kernel.org/r/20201106035911.1942128-14-harshadshirwadkar@gmail.com


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