cifs: add SMB2_ioctl_init/free helpers to be used with compounding

Define an _init() and a _free() function for SMB2_init so that we will
be able to use it with compounds.

Signed-off-by: Ronnie Sahlberg <lsahlber@redhat.com>
Signed-off-by: Steve French <stfrench@microsoft.com>