smb: client: improve compound padding in encryption

After commit f7f291e1 ("cifs: fix oops during encryption"), the
encryption layer can handle vmalloc'd buffers as well as kmalloc'd
buffers, so there is no need to inefficiently squash request iovs
into a single one to handle padding in compound requests.

Cc: David Howells <dhowells@redhat.com>
Signed-off-by: Paulo Alcantara (Red Hat) <pc@manguebit.com>
Signed-off-by: Steve French <stfrench@microsoft.com>