ext4: fast commit: add lock_updates tracepoint

ext4: fast commit: add lock_updates tracepoint

Commit-time fast commit snapshots run under jbd2_journal_lock_updates(),
so it is useful to quantify the time spent with updates locked and to
understand why snapshotting can fail.

Add a new tracepoint, ext4_fc_lock_updates, reporting the time spent in
the updates-locked window along with the number of snapshotted inodes
and ranges. Record the first snapshot failure reason in a stable snap_err
field for tooling.

Signed-off-by: Li Chen <chenl311@chinatelecom.cn>
Reviewed-by: Steven Rostedt (Google) <rostedt@goodmis.org>
Link: https://patch.msgid.link/20260515091829.194810-7-me@linux.beauty
Signed-off-by: Theodore Ts'o <tytso@mit.edu>