lib: min_heap: Refactor heapify_up/down functions

Remove using `swap()` which was using VLA and replace it
with `byteswp()`.
Reduce the scope of local variables in `heapify_up/down` to
the smallest necessary block.
Replace `while(true)` loop in `heapify_down()` with a bounded `for`
loop.

Signed-off-by: Sayooj K Karun <sayooj@aerlync.com>