Filesystem: warn if mount GFS2 with SELinux but no 'context' in options

Even GFS2 supports xattr but SELinux attempts to read sclabel element on
each file system object. It will cause a small performance drop due to
lock mechanism of GFS2 inode and xattr.

It's recommended for user to add 'context=' in options so disk reads of
extended attribute block can be avoided.
Here warn it if FSTYPE is GFS2 and SELinux is permissive/enforcing.

Signed-off-by: Su Yue <glass.su@suse.com>