bus: fix missing macro argument renaming

<fdo-vcs> systemd kay master * b1454bf src/libsystemd-bus/ bus-kernel.c kdbus.h
<fdo-vcs> systemd bus: catch up with kernel changes
<kmacleod> kay: randomly looked at your commit, it looks like in KDBUS_FOREACH_ITEM
  you missed changing a (d) to an (i) in (uint8_t*) (d) < (uint8_t*) (k) + (k)->size;  ?
<kay> kmacleod: hah, so there *is* a reason for using _foo in macros :)
<kay> kmacleod: thanks!