jbd2: update locking documentation for transaction_t

The following members of struct transaction_s aka transaction_t
were turned into lock-free variables in the past:
- t_updates
- t_outstanding_credits
- t_handle_count
However, the documentation has not been updated yet.
This commit replaced the annotated lock by [none].

Found by LockDoc (Alexander Lochmann, Horst Schirmeier and Olaf Spinczyk)

Signed-off-by: Alexander Lochmann <alexander.lochmann@tu-dortmund.de>
Signed-off-by: Horst Schirmeier <horst.schirmeier@tu-dortmund.de>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>