OpenZFS 9080 - recursive enter of vdev_indirect_rwlock from vdev_indirect_remap()

Authored by: Serapheim Dimitropoulos <serapheim.dimitro@delphix.com>
Reviewed by: Matthew Ahrens <mahrens@delphix.com>
Reviewed by: George Wilson <george.wilson@delphix.com>
Approved by: Hans Rosenfeld <rosenfeld@grumpf.hope-2000.org>
Ported-by: Brian Behlendorf <behlendorf1@llnl.gov>

OpenZFS-issue: https://illumos.org/issues/9080
OpenZFS-commit: https://github.com/openzfs/openzfs/commit/bdfded42e6
Closes #6900