Update outdated references to SLRU ControlLock

Hi,

I just noticed that some references so the now removed SLRU ControlLock were
still present in slru.[ch].  I tried to improve the situation with the attached
patch by using the new "bank control lock" name that is used nearby.

Note that the main comment of slru.c still has one paragraph that mentions
"bank control lock" consistently before switching to just "control lock" in the
next paragraph.  I'm assuming that it's ok in that context as it seems clear to
me that those are the same thing, just spelled with a less verbose name.