vhost/scsi: Use GFP_ATOMIC with percpu_ida_alloc for obtaining tag

Fix GFP_KERNEL -> GFP_ATOMIC usage of percpu_ida_alloc() within
vhost_scsi_get_tag(), as this code is expected to be called directly
from interrupt context.

v2 changes:

  - Handle possible tag < 0 failure with GFP_ATOMIC

Acked-by: Michael S. Tsirkin <mst@redhat.com>
Acked-by: Asias He <asias@redhat.com>
Cc: Kent Overstreet <kmo@daterainc.com>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>