remove use of BpfMap(fd=createMap()) construction in favour of better one

Long term I'd like to eliminate BpfMap(fd) constructor entirely.

Test: builds, atest
Signed-off-by: Maciej Żenczykowski <maze@google.com>
Change-Id: I3b60d1c94205ea9ed627d7cdee8a4b6c89a3047f