target: fix memory leak on XCOPY

On each processed XCOPY command, two "kmalloc-512" memory objects are
leaked. These represent two allocations of struct xcopy_pt_cmd in
target_core_xcopy.c.

The reason for the memory leak is that the cmd_kref field is not
initialized (thus, it is zero because the allocations were done with
kzalloc). When we decrement zero kref in target_put_sess_cmd, the result
is not zero, thus target_release_cmd_kref is not called.

This patch fixes the bug by moving kref initialization from
target_get_sess_cmd to transport_init_se_cmd (this function is called from
target_core_xcopy.c, so it will correctly initialize cmd_kref). It can be
easily verified that all code that calls target_get_sess_cmd also calls
transport_init_se_cmd earlier, thus moving kref_init shouldn't introduce
any new problems.

Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Cc: stable@vger.kernel.org	# 3.12+
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>