cifs: Fix inconsistent IS_ERR and PTR_ERR

Fix inconsistent IS_ERR and PTR_ERR in cifs_find_swn_reg(). The proper
pointer to be passed as argument to PTR_ERR() is share_name.

This bug was detected with the help of Coccinelle.

Fixes: bf80e5d4 ("cifs: Send witness register and unregister commands to userspace daemon")
Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Reviewed-by: Samuel Cabrero <scabrero@suse.de>
Signed-off-by: Steve French <stfrench@microsoft.com>