Smack: remove set but not used variable 'root_inode'

Fixes gcc '-Wunused-but-set-variable' warning:

security/smack/smackfs.c: In function 'smk_fill_super':
security/smack/smackfs.c:2856:16: warning:
 variable 'root_inode' set but not used [-Wunused-but-set-variable]
Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: Casey Schaufler <casey@schaufler-ca.com>

security/smack/smackfs.c

@@ -2853,7 +2853,6 @@ static const struct file_operations smk_ptrace_ops = {
 static int smk_fill_super(struct super_block *sb, void *data, int silent)
 {
 	int rc;
-	struct inode *root_inode;
 
 	static const struct tree_descr smack_files[] = {
 		[SMK_LOAD] = {
@@ -2917,8 +2916,6 @@ static int smk_fill_super(struct super_block *sb, void *data, int silent)
 		return rc;
 	}
 
-	root_inode = d_inode(sb->s_root);
-
 	return 0;
 }