Fix 'zpool clear' on suspended pools

Fix 'zpool clear' on suspended pools

'zpool clear' should be able to resume I/O on suspended, but otherwise
healthy, pools.

4a283c7 accidentally introduced a new code path where we call
txg_wait_synced() on the suspended pool before we had the chance to
resume I/O via zio_resume(): this results in the 'zpool clear'
command hanging indefinitely, waiting for a TXG that cannot be synced.

Fix this by avoiding the call to txg_wait_synced().

Reviewed-by: George Melikov <mail@gmelikov.ru>
Reviewed-by: loli10K <ezomori.nozomu@gmail.com>
Reviewed-by: Tony Hutter <hutter2@llnl.gov>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #6399