Only trigger SET_ERROR tracepoint event on error

Currently, the SET_ERROR tracepoint triggers regardless of whether there
is an error or not. On Illumos, SET_ERROR only triggers on an actual
error, which is avoids irrelevant noise. Linux 2.6.38 added support for
conditional tracepoints, so we modify SET_ERROR to use them when they
are avaliable for functionality equivalent to the Illumos functionality.

Signed-off-by: Richard Yao <ryao@gentoo.org>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #4043