MFV r329803:
9080 recursive enter of vdev_indirect_rwlock from vdev_indirect_remap()

illumos/illumos-gate@bdfded42e66b9fc1395ff2401aa2952f7c44ae34

A scenario came up where a callback executed by vdev_indirect_remap() on a vdev, calls
vdev_indirect_remap() on the same vdev and tries to reacquire vdev_indirect_rwlock that
was already acquired from the first call to vdev_indirect_remap(). The specific scenario,
is that we want to remap a block pointer that is snapshoted but its dataset's remap_deadlist
is not cached. So in order to add it we issue a read through a vdev_indirect_remap() on the
same vdev, which brings up the aforementioned issue.

Reviewed by: Matthew Ahrens <mahrens@delphix.com>
Reviewed by: George Wilson <george.wilson@delphix.com>
Approved by: Hans Rosenfeld <rosenfeld@grumpf.hope-2000.org>
Author: Serapheim Dimitropoulos <serapheim.dimitro@delphix.com>