[patch] Fix crash on userland fbt tracing

[Mark Johnston]

On Mon, Apr 11, 2016 at 07:05:01PM +0900, Jihyun Yu wrote:
> How to reproduce crash (tested on 10.3)
> 
> - Run `top` on a shell
> 
> - List dtrace probes with following command:
> dtrace -p `pgrep top` -ln 'pid$target::clear:entry {}
> pid$target::clear:return {}'
> 
> - Try to attach probes without specifing pid provider
> # dtrace -p `pgrep top` -n '::clear:entry {} ::clear:return {}'
> Assertion failed: (err == 0), file
> /usr/src/cddl/lib/libdtrace/../../../cddl/contrib/opensolaris/lib/libdtrace/common/dt_proc.c,
> line 1131.
> 
> 
> The crash is caused by a deadlock, and here's a fix.

This was committed as r297827. Thanks!