target: Clean up returning errors in PR handling code

 - instead of (PTR_ERR(file) < 0) just use IS_ERR(file)
 - return -EINVAL instead of EINVAL
 - all other error returns in target_scsi3_emulate_pr_out() use
   "goto out" -- get rid of the one remaining straight "return."

Signed-off-by: Roland Dreier <roland@purestorage.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>