sched_ext: Rename CFI stubs to names that are recognized by BPF

CFI stubs can be used to tag arguments with __nullable (and possibly other
tags in the future) but for that to work the CFI stubs must have names that
are recognized by BPF. Rename them.

Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: David Vernet <void@manifault.com>
Acked-by: Martin KaFai Lau <martin.lau@kernel.org>
Acked-by: Alexei Starovoitov <ast@kernel.org>