cifs: Fix an error pointer dereference in cifs_mount()

The error handling calls kfree(full_path) so we can't let it be a NULL
pointer.  There used to be a NULL assignment here but we accidentally
deleted it.  Add it back.

Fixes: 7efd0815 ("cifs: document and cleanup dfs mount")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Steve French <stfrench@microsoft.com>
Reviewed-by: Paulo Alcantara (SUSE) <pc@cjr.nz>