elf/cache.c: Fix resource leaks identified by static analyzers

A coverity run identified a number of resource leaks in cache.c.
There are a couple of simple memory leaks where a local allocation is
not freed before function return.  Then there is a mmap leak and a
file descriptor leak where a map is not unmapped in the error case and
a file descriptor remains open respectively.

Reviewed-by: Adhemerval Zanella <adhemerval.zanella@linaro.org>