volume_id: add f2fs detection

function                                             old     new   delta
volume_id_probe_f2fs                                   -      98     +98

Signed-off-by: Sven-Göran Bergh <sgb@systemasis.org>

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>
Fix build with latest AOSP.

Obviously this isn't pretty and needs to be addressed...