ovl: check privs before decoding file handle

ovl: check privs before decoding file handle

CAP_DAC_READ_SEARCH is required by open_by_handle_at(2) so check it in
ovl_decode_real_fh() as well to prevent privilege escalation for
unprivileged overlay mounts.

[Amir] If the mounter is not capable in init ns, ovl_check_origin() and
ovl_verify_index() will not function as expected and this will break index
and nfs export features.  So check capability in ovl_can_decode_fh(), to
auto disable those features.

Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>