zfs: fix vdev_rebuild_thread deadlock

zfs: fix vdev_rebuild_thread deadlock

From the openzfs 8e43fa12c commit message:
  The metaslab_disable() call may block waiting for a txg sync.
  Therefore it's important that vdev_rebuild_thread release the
  SCL_CONFIG read lock it is holding before this call.  Failure
  to do so can result in the txg_sync thread getting blocked
  waiting for this lock which results in a deadlock.

Obtained from:	openzfs/zfs@8e43fa12c571878f32119122771888ab9092a75a
Approved by:	re (delphij)

(cherry picked from commit c7069063144395b025c0ef178431eb2cafdbe1ca)