rtio: fix bug in mempool release API

It was previously assumed that the 'sys_mem_blocks' struct would maintain
information about contiguous blocks allocated so the release API only
took the starting address. This led to an issue where allocating 2+
blocks would end up with a memory leak because any block not being the
first would never be released.

Add the buffer length as an argument so the correct number of blocks can
be released. Also, ammend the tests to match and verify.

Signed-off-by: Yuval Peress <peress@google.com>