fs: cifs: remove unused variable.

In SMB2_set_compression(), the "res_key" variable is only initialized to NULL
and later kfreed. It is therefore useless and should be removed.

Found with the following semantic patch:

<smpl>
@@
identifier foo;
identifier f;
type T;
@@
* f(...) {
...
* T *foo = NULL;
... when forall
    when != foo
* kfree(foo);
...
}
</smpl>

Signed-off-by: Cyril Roelandt <tipecaml@gmail.com>
Signed-off-by: Steve French <sfrench@us.ibm.com>