SELinux: fix array out of bounds when mounting with selinux options

Given an illegal selinux option it was possible for match_token to work in
random memory at the end of the match_table_t array.

Note that privilege is required to perform a context mount, so this issue is
effectively limited to root only.

Signed-off-by: Eric Paris <eparis@redhat.com>
Acked-by: Stephen Smalley <sds@tycho.nsa.gov>
Signed-off-by: James Morris <jmorris@namei.org>