jbd2: fix lockdep annotation in add_transaction_credits()

Thomas has reported a lockdep splat hitting in
add_transaction_credits(). The problem is that that function calls
jbd2_might_wait_for_commit() while holding j_state_lock which is wrong
(we do not really wait for transaction commit while holding that lock).

Fix the problem by moving jbd2_might_wait_for_commit() into places where
we are ready to wait for transaction commit and thus j_state_lock is
unlocked.

Cc: stable@vger.kernel.org
Fixes: 1eaa566d
Reported-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Jan Kara <jack@suse.cz>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>