ext4: fast commit: avoid i_data_sem by dropping ext4_map_blocks() in snapshots

Commit-time snapshots run under jbd2_journal_lock_updates(), so the work
done there must stay bounded.

The snapshot path still used ext4_map_blocks() to build data ranges. This
can take i_data_sem and pulls the mapping code into the snapshot logic.
Build inode data range snapshots from the extent status tree instead.

The extent status tree is a cache, not an authoritative source. If the
needed information is missing or unstable (e.g. delayed allocation), treat
the transaction as fast commit ineligible and fall back to full commit.

Also cap the number of inodes and ranges snapshotted per fast commit and
allocate range records from a dedicated slab cache. The inode pointer
array is allocated outside the updates-locked window.

Testing: QEMU/KVM guest, virtio-pmem + dax, ext4 -O fast_commit, mounted
dax,noatime. Ran python3 500x {4K write + fsync}, fallocate 256M, and
python3 500x {creat + fsync(dir)} without lockdep splats or errors.

Signed-off-by: Li Chen <chenl311@chinatelecom.cn>
Link: https://patch.msgid.link/20260515091829.194810-6-me@linux.beauty
Signed-off-by: Theodore Ts'o <tytso@mit.edu>

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