Don't abuse vfork()

According to POSIX.1, "vfork() has the same effect as fork(2),
except that the behavior is undefined if the process created by vfork()
either modifies any data other than a variable of type pid_t
used to store the return value from vfork(), [...],
or calls any other function before successfully calling _exit(2)
or one of the exec(3) family of functions."

These do all three, and work by pure chance
(or maybe they don't, but we blisfully don't know).
Either way: bad idea to call vfork() from C,
unless you're the standard library, and POSIX.1-2008 removes it entirely

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Ahelenia Ziemiańska <nabijaczleweli@nabijaczleweli.xyz>
Closes #12015

tests/zfs-tests/cmd/xattrtest/xattrtest.c

@@ -262,7 +262,7 @@ run_process(const char *path, char *argv[])
 	pid_t pid;
 	int rc, devnull_fd;
 
-	pid = vfork();
+	pid = fork();
 	if (pid == 0) {
 		devnull_fd = open("/dev/null", O_WRONLY);