LU-2224 osd-zfs: Fix osd_commit_async() locking

LU-2224 osd-zfs: Fix osd_commit_async() locking

The ZFS osd_commit_async() function never properly acquires the
tx->tx_sync_lock() mutex to protext the tx_state_t.  However,
the mutex is correctly dropped so we just add the obviously
missing mutex_enter().

Change-Id: Iae426feaeb5885034515d6bf0ccb9509ed098bb0
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-on: http://review.whamcloud.com/4383
Tested-by: Hudson
Reviewed-by: Alex Zhuravlev <bzzz@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Prakash Surya <surya1@llnl.gov>
Tested-by: Maloo <whamcloud.maloo@gmail.com>