MAINTAINERS: add kernel/fork.c to relevant sections

Currently kernel/fork.c both contains absolutely key logic relating to a
number of kernel subsystems and also has absolutely no assignment in
MAINTAINERS.

Correct this by placing this file in relevant sections - mm core, exec and
the scheduler so people know who to contact when making changes here.

scripts/get_maintainers.pl can perfectly well handle a file being in
multiple sections, so this functions correctly.

Intent is that we keep putting changes to kernel/fork.c through Andrew's
tree.

Link: https://lkml.kernel.org/r/20250513145706.122101-1-lorenzo.stoakes@oracle.com
Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: Michal Hocko <mhocko@suse.com>
Acked-by: Vlastimil Babka <vbabka@suse.cz>
Reviewed-by: Kees Cook <kees@kernel.org>
Acked-by: David Hildenbrand <david@redhat.com>
Acked-by: Liam R. Howlett <Liam.Howlett@oracle.com>
Cc: Ben Segall <bsegall@google.com>
Cc: Dietmar Eggemann <dietmar.eggemann@arm.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Juri Lelli <juri.lelli@redhat.com>
Cc: Mel Gorman <mgorman <mgorman@suse.de>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Valentin Schneider <vschneid@redhat.com>
Cc: Vincent Guittot <vincent.guittot@linaro.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

Author: ljskernel

MAINTAINERS

@@ -8840,6 +8840,7 @@ F:	include/linux/elf.h
 F:	include/uapi/linux/auxvec.h
 F:	include/uapi/linux/binfmts.h
 F:	include/uapi/linux/elf.h
+F:	kernel/fork.c
 F:	mm/vma_exec.c
 F:	tools/testing/selftests/exec/
 N:	asm/elf.h
@@ -15540,6 +15541,7 @@ F:	include/linux/mm.h
 F:	include/linux/mm_*.h
 F:	include/linux/mmdebug.h
 F:	include/linux/pagewalk.h
+F:	kernel/fork.c
 F:	mm/Kconfig
 F:	mm/debug.c
 F:	mm/init-mm.c
@@ -21770,6 +21772,7 @@ F:	include/linux/preempt.h
 F:	include/linux/sched.h
 F:	include/linux/wait.h
 F:	include/uapi/linux/sched.h
+F:	kernel/fork.c
 F:	kernel/sched/
 
 SCHEDULER - SCHED_EXT