OpenZFS 6569 - large file delete can starve out write ops

Authored by: Alek Pinchuk <alek@nexenta.com>
Reviewed by: Matt Ahrens <mahrens@delphix.com>
Reviewed by: Sanjay Nadkarni <sanjay.nadkarni@nexenta.com>
Reviewed by: Pavel Zakharov <pavel.zakharov@delphix.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Ported-by: George Melikov <mail@gmelikov.ru>
Tested-by: kernelOfTruth <kerneloftruth@gmail.com>

OpenZFS-issue: https://www.illumos.org/issues/6569
OpenZFS-commit: https://github.com/openzfs/openzfs/commit/1bf4b6f2
Closes #5706