smb: client: Use snprintf in cifs_set_cifscreds

smb: client: Use snprintf in cifs_set_cifscreds

Replace unbounded sprintf() calls with the safer snprintf(). Avoid using
magic numbers and use strlen() to calculate the key descriptor buffer
size. Save the size in a local variable and reuse it for the bounded
snprintf() calls. Remove CIFSCREDS_DESC_SIZE.

Signed-off-by: Thorsten Blum <thorsten.blum@linux.dev>
Acked-by: Paulo Alcantara (Red Hat) <pc@manguebit.org>
Signed-off-by: Steve French <stfrench@microsoft.com>