ext4: temporarily elevate commit thread priority

ext4: temporarily elevate commit thread priority

Unlike JBD2 based full commits, there is no dedicated journal thread
for fast commits. Thus to reduce scheduling delays between IO
submission and completion, temporarily elevate the committer thread's
priority to match the configured priority of the JBD2 journal
thread.

Signed-off-by: Harshad Shirwadkar <harshadshirwadkar@gmail.com>
Reviewed-by: Jan Kara <jack@suse.cz>
Link: https://patch.msgid.link/20250508175908.1004880-8-harshadshirwadkar@gmail.com
Signed-off-by: Theodore Ts'o <tytso@mit.edu>