iscsi-target: move iscsit_thread_check_cpumask()

Move iscsit_thread_check_cpumask() to header
file so that ISCSI_HW_OFFLOAD and other transport drivers
can call this function to ensure both tx and rx thread
runs on same cpu.

Signed-off-by: Varun Prakash <varun@chelsio.com>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>