Fix race in libzfs_run_process_impl

When replacing a disk, a child process is forked to run a script called
zfs_prepare_disk (which can be useful for disk firmware update or health
check). By default this script does nothing - it simply returns 0.

When testing on a virtual machine, it returns so fast that the parent
misses it: when checking, the child already exited. As waitpid returns
-1, the parent incorrectly assume that the child process had an error
or was killed. This, in turn, leaves the newly added disk in REMOVED or
UNAVAIL status rather than completing the replace process.

As child should be inspected via waitpid status flag and the relative
macros, this patch remove the check around waitpid return code.

Signed-off-by: Gionatan Danti <g.danti@assyoma.it>

lib/libzfs/libzfs_util.c

@@ -967,7 +967,7 @@ libzfs_run_process_impl(const char *path, char *argv[], char *env[], int flags,
 		while ((error = waitpid(pid, &status, 0)) == -1 &&
 		    errno == EINTR)
 			;
-		if (error < 0 || !WIFEXITED(status))
+		if (!WIFEXITED(status))
 			return (-1);
 
 		if (lines != NULL) {