OpenZFS 6637 - replacing "dontclose" with "should_close"

OpenZFS 6637 - replacing "dontclose" with "should_close"

Authored by: David Schwartz <dschwartz783@gmail.com>
Reviewed by: Matthew Ahrens <mahrens@delphix.com>
Reviewed by: Prakash Surya <prakash.surya@delphix.com>
Approved by: Robert Mustacchi <rm@joyent.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Giuseppe Di Natale <dinatale2@llnl.gov>
Ported-by: George Melikov <mail@gmelikov.ru>
I find that this is a lot easier to read. "not don't close" is somewhat tough on the eyes.

OpenZFS-issue: https://www.illumos.org/issues/6637
OpenZFS-commit: https://github.com/openzfs/openzfs/commit/d189620
Closes #5572