jbd2: Improve scalability by not taking j_state_lock in jbd2_journal_stop()

One of the most contended locks in the jbd2 layer is j_state_lock when
running dbench.  This is especially true if using the real-time kernel
with its "sleeping spinlocks" patch that replaces spinlocks with
priority inheriting mutexes --- but it also shows up on large SMP
benchmarks.

Thanks to John Stultz for pointing this out.

Reviewed by Mingming Cao and Jan Kara.

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