mem-cache: Make checking function const in FALRU

mem-cache: Make checking function const in FALRU

The checking function should not be able to modify either
the head and tail pointers nor should it modify its class.

Change-Id: I2ad495f0c8c6b778d48512143e94b4c9a353f22e
Signed-off-by: Daniel R. Carvalho <odanrc@yahoo.com.br>
Reviewed-on: https://gem5-review.googlesource.com/c/13209
Reviewed-by: Nikos Nikoleris <nikos.nikoleris@arm.com>
Maintainer: Nikos Nikoleris <nikos.nikoleris@arm.com>