ext4: use ext4_journal_start/stop for fast commit transactions

ext4: use ext4_journal_start/stop for fast commit transactions

This patch drops all calls to ext4_fc_start_update() and
ext4_fc_stop_update(). To ensure that there are no ongoing journal
updates during fast commit, we also make jbd2_fc_begin_commit() lock
journal for updates. This way we don't have to maintain two different
transaction start stop APIs for fast commit and full commit. This
patch doesn't remove the functions altogether since in future we want
to have inode level locking for fast commits.

Signed-off-by: Harshad Shirwadkar <harshadshirwadkar@gmail.com>
Link: https://lore.kernel.org/r/20211223202140.2061101-2-harshads@google.com
Signed-off-by: Theodore Ts'o <tytso@mit.edu>