gfs2_grow/jadd: Use a matching context mount option in mount_gfs2_meta
    
    On a system with SELinux enabled, if a gfs2 file system is mounted with
    a context= option, the tools gfs2_grow and gfs2_jadd will fail with
    "Invalid argument". This is due to SELinux failing the mount due to an
    mismatched context ("SELinux: mount invalid.  Same superblock, different
    security settings").
    
    In order to work around this, parse the context option of the mntent
    relating to the gfs2 mount point and use it in mount_gfs2_meta().
    
    Signed-off-by: Andrew Price <anprice@redhat.com>