tcm_loop: fixup tpgt string to integer conversion

Currently, for example, mkdir "tpgt_xyz" doesn't return error.
mkdir /sys/kernel/config/target/loopback/naa.60014055f195952b/tpgt_xyz

Replace obsoleted simple_strtoul with kstrtoul and check the conversion.

Signed-off-by: Ming Lin <mlin@kernel.org>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>