Remove unnecessary ERR_PTR()/PTR_ERR() cast in proc_setup_thread_self().

Link: http://lkml.kernel.org/r/20190124030150.8472-2-cgxu519@gmx.comSigned-off-by: Chengguang Xu <cgxu519@gmx.com>
Reviewed-by: Andrew Morton <akpm@linux-foundation.org>
Cc: Alexey Dobriyan <adobriyan@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>