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>

drivers/vhost/scsi.c

@@ -728,7 +728,12 @@ vhost_scsi_get_tag(struct vhost_virtqueue *vq,
 	}
 	se_sess = tv_nexus->tvn_se_sess;
 
-	tag = percpu_ida_alloc(&se_sess->sess_tag_pool, GFP_KERNEL);
+	tag = percpu_ida_alloc(&se_sess->sess_tag_pool, GFP_ATOMIC);
+	if (tag < 0) {
+		pr_err("Unable to obtain tag for tcm_vhost_cmd\n");
+		return ERR_PTR(-ENOMEM);
+	}
+
 	cmd = &((struct tcm_vhost_cmd *)se_sess->sess_cmd_map)[tag];
 	sg = cmd->tvc_sgl;
 	pages = cmd->tvc_upages;