fix format problems in tar.c

fix format problems in tar.c

%o is unsigned, but off_t is signed.
* takes an int.
an error_msg call was missing an argument.

only one of these is an actual error, but i'd like to fix the others
too so that we (toybox, but if not, then Android) can turn on format
string warnings to prevent future bugs like the stat.c LP32 ones.