mptcp: pm: Return local variable instead of freed pointer

mptcp: pm: Return local variable instead of freed pointer

Commit e4c28e3d5c090 ("mptcp: pm: move generic PM helpers to pm.c")
removed an unnecessary if-check, which resulted in returning a freed
pointer.

This still works due to the implicit boolean conversion when returning
the freed pointer from mptcp_remove_anno_list_by_saddr(), but it can be
confusing and potentially error-prone. To improve clarity, add a local
variable to explicitly return a boolean value instead.

Signed-off-by: Thorsten Blum <thorsten.blum@linux.dev>
Reviewed-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Link: https://patch.msgid.link/20250413-net-next-mptcp-sched-mib-sft-misc-v2-3-0f83a4350150@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>