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>