parasite: heap engine update

1) Use macro for defining size mapped size.
2) Allow to allocate all space (including the last byte).

Signed-off-by: Stanislav Kinsbursky <skinsbursky@openvz.org>
Acked-by: Pavel Emelyanov <xemul@parallels.com>
[gorcunov@: MAX_BUF_SIZE tuneup, don't use ops without braces)]
Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>