cifs: fix length handling in cifs_get_name_from_search_buf

The earlier patch to move this code to use the new unicode helpers
assumed that the filename strings would be null terminated. That's not
always the case.

Instead of passing "max_len" to the string converter, pass "min(len,
max_len)", which makes it do the right thing while still keeping the
parser confined to the response. Also fix up the prototypes of this
function and the callers so that max_len is unsigned (like len is).

Signed-off-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: Steve French <sfrench@us.ibm.com>