Fix overly broad spa config lock

The spa_txg_history_init_io() and spa_txg_history_fini_io() were
mistakenly taking SCL_ALL when only SCL_CONFIG is required to
access the vdev stats.  This could result in a deadlock which
was observed when running ztest.

Reviewed-by: Olaf Faaland <[email protected]>
Reviewed-by: Tim Chase <[email protected]>
Signed-off-by: Brian Behlendorf <[email protected]>
Closes #8445 

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