target: replace strict_strto*() with kstrto*()

The usage of strict_strtoul() and strict_strtoull() is not preferred,
because strict_strtoul() and strict_strtoull() are obsolete. Thus,
kstrtoul() and kstrtoull() should be used.

v2: Fix incorrect return in ft_add_tpg (Fengguang)

Signed-off-by: Jingoo Han <jg1.han@samsung.com>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>