Fix race in trace point in zrl_add_impl

We hit an illegal memory access in the zrlock trace point. The problem
is that zrl->zr_owner and zrl->zr_caller are assigned locklessly. And if
zrl->zr_owner got assigned a longer string between when string()
calculate the strlen, and when assign_str() does strcpy. The copy will
overflow the buffer.

For example:

Initial condition:
zrl->zr_owner = A
zrl->zr_caller = "abc"

Thread A Thread B

if (zrl->zr_owner == A) {

  DTRACE_PROBE2() {
    __string() {
      strlen(zrl->zr_caller) -> 3
      allocate buf[4]
    }

                                        zrl->zr_owner = B
				        zrl->zr_caller = "abcd"

    __assign_str() {
      strcpy(buf, zrl->zr_caller) <- buffer overflow

Dereferencing zrl->zr_owner->pid may also be problematic, in that the
zrl->zr_owner got changed to other task, and that task exits, freeing
the task_struct. This should be very unlikely, as the other task need to
zrl_remove and exit between the dereferencing zr->zr_owner and
zr->zr_owner->pid. Nevertheless, we'll deal with it as well.

To fix the zrl->zr_caller issue, instead of copy the string content, we
just copy the pointer, this is safe because it always points to
func, which is static. As for the zrl->zr_owner issue, we pass in
curthread instead of using zrl->zr_owner.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Chunwei Chen <david.chen@nutanix.com>
Closes #7291