cifs: Use GFP_ATOMIC when a lock is held in cifs_mount()

A spin lock is held before kstrndup, it may sleep with holding
the spinlock, so we should use GFP_ATOMIC instead.

Fixes: e58c31d5e387 ("cifs: Add support for failover in cifs_reconnect()")
Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: Steve French <stfrench@microsoft.com>
Reviewed-by: Paulo Alcantara <palcantara@suse.de>