iscsi-target: Convert gfp_t parameter to task state bitmask

This patch propigates the use of task state bitmask now used by
percpu_ida_alloc() up the iscsi-target callchain, replacing the
use of GFP_ATOMIC for TASK_RUNNING, and GFP_KERNEL for
TASK_INTERRUPTIBLE.

Also, drop the unnecessary gfp_t parameter to isert_allocate_cmd(),
and just pass TASK_INTERRUPTIBLE into iscsit_allocate_cmd().

Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>