ext4: fast commit: snapshot inode state before writing log

Fast commit writes inode metadata and data range updates after unlocking
journal updates. New handles can start at that point, so the log writing
path must not look at live inode state.

Add a commit-time per-inode snapshot and populate it while journal updates
are locked and existing handles are drained. Store the snapshot behind
ext4_inode_info->i_fc_snap so ext4_inode_info only grows by one pointer.
The snapshot contains a copy of the on-disk inode plus the data range
records needed for fast commit TLVs.

Snapshotting runs under jbd2_journal_lock_updates(). Avoid triggering I/O
there by using ext4_get_inode_loc_noio() and falling back to full commit
if the inode table block is not present or not uptodate.

Log writing then only serializes the snapshot, so it no longer needs to
call ext4_map_blocks() and take i_data_sem under s_fc_lock. The snapshot
is installed and freed under s_fc_lock and is released from fast commit
cleanup and inode eviction.

Signed-off-by: Li Chen <chenl311@chinatelecom.cn>
Link: https://patch.msgid.link/20260515091829.194810-2-me@linux.beauty
Signed-off-by: Theodore Ts'o <tytso@mit.edu>

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