9p/xen: Fix end of loop tests for list_for_each_entry

This patch addresses the following problems:
 - priv can never be NULL, so this part of the check is useless
 - if the loop ran through the whole list, priv->client is invalid and
it is more appropriate and sufficient to check for the end of
list_for_each_entry loop condition.

Link: http://lkml.kernel.org/r/20210727000709.225032-1-harshvardhan.jha@oracle.com


Signed-off-by: Harshvardhan Jha <harshvardhan.jha@oracle.com>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>
Tested-by: Stefano Stabellini <sstabellini@kernel.org>
Cc: <stable@vger.kernel.org>
Signed-off-by: Dominique Martinet <asmadeus@codewreck.org>