mm: Kconfig: group swap, slab, hotplug and thp options into submenus

mm: Kconfig: group swap, slab, hotplug and thp options into submenus

There are several clusters of related config options spread throughout the
mostly flat MM submenu.  Group them together and put specialization
options into further subdirectories to make the MM submenu a bit more
organized and easier to navigate.

[[email protected]: fix kbuild warnings]
Link: https://lkml.kernel.org/r/[email protected]
[[email protected]: fix more kbuild warnings]
Link: https://lkml.kernel.org/r/[email protected]
Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Johannes Weiner <[email protected]>
Cc: Dan Streetman <[email protected]>
Cc: Michal Hocko <[email protected]>
Cc: Minchan Kim <[email protected]>
Cc: Roman Gushchin <[email protected]>
Cc: Seth Jennings <[email protected]>
Cc: Shakeel Butt <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>