MAINTAINERS: add an entry for collapse

Collapse is now its own body of code: mm/collapse.c, a header stating what
it offers a caller, and its own tracepoint family.  It has two callers,
the khugepaged daemon and MADV_COLLAPSE, rather than living inside one of
them.

Add a section for it, and move what belongs to it out of the THP entry:
khugepaged.c and its header, the trace header, the selftests, and
mm_slot.h.

The selftests are matched as khugepaged*.c, which also covers the race and
sync-check harnesses the THP entry did not list.  mm_slot.h stays listed
under KSM as well, which is the other user of it.

Assisted-by: Claude-Code:claude-opus-5
Signed-off-by: Kiryl Shutsemau (Meta) <kas@kernel.org>
(cherry picked from commit 167fcec206c1d0695abc7875a7b65140b8f04e81)