jbd2: calculate statistics without holding j_state_lock and j_list_lock

The two hottest locks, and thus the biggest scalability bottlenecks,
in the jbd2 layer, are the j_list_lock and j_state_lock.  This has
inspired some people to do some truly unnatural things[1].

[1] https://www.usenix.org/system/files/conference/fast14/fast14-paper_kang.pdf

We don't need to be holding both j_state_lock and j_list_lock while
calculating the journal statistics, so move those calculations to the
very end of jbd2_journal_commit_transaction.

Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>

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