Revert "Fix __zio_execute() asynchronous dispatch"

This reverts commit 91579709fccd3e55a21970742b66c388fb1403db which
limited the asynchronous dispatch to kernel space.  We want to do
this for two reasons:

1) While we have slightly more headroom in user space excessively
   deep stacks have been observed while running ztest, see #2293.

2) Removing this conditional makes the pipeline behave consistently
   regardless of if it's executing in kernel space or user space.
   This way we're more likely to uncover subtle issues with ztest.

Signed-off-by: Richard Yao <ryao@gentoo.org>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #2384
Accept udev and dracut paths specified by ./configure

There are two common locations where udev and dracut components are
commonly installed.  When building packages using the 'make rpm|deb'
targets check those common locations and pass them to rpmbuild.  For
non-standard configurations these values can be provided by the
the following configure options:

  --with-udevdir=DIR      install udev helpers [default=check]
  --with-udevruledir=DIR  install udev rules [[UDEVDIR/rules.d]]
  --with-dracutdir=DIR    install dracut helpers [default=check]

When rebuilding using the source packages the per-distribution
default values specified in the spec file will be used.  This is
the preferred way to build packages for a distribution but the
ability to override the defaults is provided as a convenience.

Signed-off-by: Turbo Fredriksson <turbo@bayour.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #2310
Closes #1680