util: Heap-allocate 256K zlib buffer

util: Heap-allocate 256K zlib buffer

The disk cache code tries to allocate a 256 Kbyte buffer on the stack.
Since musl only gives 80 Kbyte of stack space per thread, this causes a
trap.

See https://wiki.musl-libc.org/functional-differences-from-glibc.html#Thread-stack-size

(In musl-1.1.21 the default stack size has increased to 128K)

[mattst88]: Original author unknown, but I think this is small enough
            that it is not copyrightable.
Reviewed-by: Matt Turner <mattst88@gmail.com>
Reviewed-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Eric Engestrom <eric.engestrom@intel.com>