mem: Standardize mem folder header guards

Standardize all header guards in the mem directory according to the most
frequent patterns. In general they have the form:
mem: __FOLDER_TREE_FILE_NAME_HH__
ruby: __FOLDER_TREE_FILENAME_HH__

Change-Id: I983853e292deb302becf151bf0e970057dc24774
Reviewed-on: https://gem5-review.googlesource.com/7881
Reviewed-by: Nikos Nikoleris <nikos.nikoleris@arm.com>
Maintainer: Nikos Nikoleris <nikos.nikoleris@arm.com>

ruby: Check MessageBuffer space in garnet NetworkInterface

Garnet's NetworkInterface does not consider the size of MessageBuffers when
ejecting a Message from the network. Add a size check for the MessageBuffer
and only enqueue if space is available. If space is not available, the
message if placed in a queue and the credit is held. A callback from the
MessageBuffer is implemented to wake the NetworkInterface. If there are
messages in the stalled queue, they are processed first, in a FIFO manner
and if succesfully ejected, the credit is finally sent back upstream. The
maximum size of the stall queue is equal to the number of valid VNETs
with MessageBuffers attached.

ruby: garnet2.0
Revamped version of garnet with more optimized single-cycle routers,
more configurability, and cleaner code.