Avoid dirtying the final TXGs when exporting a pool

There are two codepaths than can dirty final TXGs:

1) If calling spa_export_common()->spa_unload()->
   spa_unload_log_sm_flush_all() after the spa_final_txg is set, then
   spa_sync()->spa_flush_metaslabs() may end up dirtying the final
   TXGs. Then we have the following panic:
   Call Trace:
    <TASK>
    dump_stack_lvl+0x46/0x62
    spl_panic+0xea/0x102 [spl]
    dbuf_dirty+0xcd6/0x11b0 [zfs]
    zap_lockdir_impl+0x321/0x590 [zfs]
    zap_lockdir+0xed/0x150 [zfs]
    zap_update+0x69/0x250 [zfs]
    feature_sync+0x5f/0x190 [zfs]
    space_map_alloc+0x83/0xc0 [zfs]
    spa_generate_syncing_log_sm+0x10b/0x2f0 [zfs]
    spa_flush_metaslabs+0xb2/0x350 [zfs]
    spa_sync_iterate_to_convergence+0x15a/0x320 [zfs]
    spa_sync+0x2e0/0x840 [zfs]
    txg_sync_thread+0x2b1/0x3f0 [zfs]
    thread_generic_wrapper+0x62/0xa0 [spl]
    kthread+0x127/0x150
    ret_from_fork+0x22/0x30
    </TASK>

2) Calling vdev_*_stop_all() for a second time in spa_unload() after
   spa_export_common() unnecessarily delays the final TXGs beyond what
   spa_final_txg is set at.

Fix this by performing the check and call for
spa_unload_log_sm_flush_all() before the spa_final_txg is set in
spa_export_common(). Also check if the spa_final_txg has already been
set in spa_unload() and skip those calls in this case.

Reviewed-by: Brian Behlendorf <[email protected]>
Signed-off-by: George Amanakis <[email protected]>
External-issue: https://www.illumos.org/issues/9081
Closes openzfs#13048 
Closes openzfs#13098