Fixed a potential bug as described in openzfs#8233:

Consider this scenario (see [txg.c](https://github.com/zfsonlinux/zfs/blob/06f3fc2a4b097545259935d54634c5c6f49ed20f/module/zfs/txg.c) ):
There is heavy write load when the pool exports.
After `txg_sync_stop`'s call of `txg_wait_synced` returns, many more txgs get processed, but right before` txg_sync_stop` gets `tx_sync_lock`, the following happens:

- `txg_sync_thread` begins waiting on `tx_sync_more_cv`.
- `txg_quiesce_thread` gets done with `txg_quiesce(dp, txg)`.
- `txg_sync_stop` gets `tx_sync_lock` first, calls `cv_broadcast`s with `tx_exiting` == 1, and waits for exits.
- `txg_sync_thread` wakes up first and exits.
- Finally, `txg_quiesce_thread` gets `tx_sync_lock`, and calls `cv_broadcast(&tx->tx_sync_more_cv)`, 
but `txg_sync_thread` is already gone, and the txg in `txg_quiesce(dp, txg)` above never gets synced.

Signed-off-by: Leap Second <leapsecond@protonmail.com>

Fixed checkstyle complaints:
./module/zfs/txg.c: 558: line > 80 characters
./module/zfs/txg.c: 562: line > 80 characters

Signed-off-by: Leap Second <leapsecond@protonmail.com>

Addressed checkstype complaints:
./module/zfs/txg.c: 559: continuation should be indented 4 spaces
./module/zfs/txg.c: 564: continuation should be indented 4 spaces

Signed-off-by: Leap Second <leapsecond@protonmail.com>

Addressed checkstyle complaints:
./module/zfs/txg.c: 559: spaces instead of tabs
./module/zfs/txg.c: 559: continuation should be indented 4 spaces
./module/zfs/txg.c: 564: spaces instead of tabs
./module/zfs/txg.c: 564: continuation should be indented 4 spaces

Signed-off-by: Leap Second <leapsecond@protonmail.com>

Signed-off-by: Leap Second <leapsecond@protonmail.com>

Signed-off-by: Leap Second <leapsecond@protonmail.com>