Fix memory leak of f2fs_info

'struct f2fs_info' from system/extras/f2fs_utils is supposed to be
freed using free_f2fs_info(), not just free().

Test: see I08fc8465f7962abd698904b5466f3ed080d53953
Change-Id: If6e75e5c604b40be24538b156a37cc76f4f0d4f7