sched_ext: Move built-in idle CPU selection policy to a separate file

As ext.c is becoming quite large, move the idle CPU selection policy to
separate files (ext_idle.c / ext_idle.h) for better code readability.

Moreover, group together all the idle CPU selection kfunc's to the same
btf_kfunc_id_set block.

No functional changes, this is purely code reorganization.

Suggested-by: Yury Norov <[email protected]>
Signed-off-by: Andrea Righi <[email protected]>
Signed-off-by: Tejun Heo <[email protected]>

Author: arighi

MAINTAINERS

@@ -21006,8 +21006,7 @@ S:	Maintained
 W:	https://github.com/sched-ext/scx
 T:	git://git.kernel.org/pub/scm/linux/kernel/git/tj/sched_ext.git
 F:	include/linux/sched/ext.h
-F:	kernel/sched/ext.h
-F:	kernel/sched/ext.c
+F:	kernel/sched/ext*
 F:	tools/sched_ext/
 F:	tools/testing/selftests/sched_ext
 

kernel/sched/build_policy.c

@@ -61,6 +61,7 @@
 
 #ifdef CONFIG_SCHED_CLASS_EXT
 # include "ext.c"
+# include "ext_idle.c"
 #endif
 
 #include "syscalls.c"