selinux: fix a missing-check bug in selinux_sb_eat_lsm_opts()

In selinux_sb_eat_lsm_opts(), 'arg' is allocated by kmemdup_nul(). It
returns NULL when fails. So 'arg' should be checked. And 'mnt_opts'
should be freed when error.

Signed-off-by: Gen Zhang <blackgod016574@gmail.com>
Fixes: 99dbbb59

 ("selinux: rewrite selinux_sb_eat_lsm_opts()")
Cc: <stable@vger.kernel.org>
Signed-off-by: Paul Moore <paul@paul-moore.com>