bpf: Add a few bpf_cgroup_array_* helper functions

In the upcoming patch, the array can store a struct_ops map.
The array could have a cfi_stubs acting as a dummy instead of
the dummy_bpf_prog. The array logic will need to skip the cfi_stubs
also in order to support storing struct_ops map in the array.

bpf_cgroup_array_length(), bpf_cgroup_array_copy_to_user(), and
bpf_cgroup_array_delete_safe_at() are added as a preparation work
to allow skipping the cfi_stubs in the upcoming patch. This patch
only skips the dummy_bpf_prog which is the same as the existing behavior.
The current bpf_prog_array_*() callers are changed to call the new
bpf_cgroup_array_*(). This is a no-op change.

Unlike bpf_prog_array_copy_to_user(), bpf_cgroup_array_copy_to_user()
does not need a temporary buffer. The cgroup caller already holds
cgroup_mutex and dereferences the effective array with
rcu_dereference_protected(), so it does not copy to userspace
from an RCU read-side critical section. Details in commit 0911287ce32b.

Another addition is the bpf_cgroup_array_free(). This prepares
the array to have a different rcu gp for the struct_ops use case,
for example, a struct_ops could have mix of sleepable ops and
non-sleepable ops. In this patch, bpf_cgroup_array_free() only
goes through the regular rcu gp. This is a no-op change also.

bpf_prog_dummy() is also added to return the global dummy_bpf_prog.

bpf_cgroup_array_dummy() is added to decide the sentinel based on atype.
It now always returns bpf_prog_dummy(). In the upcoming patch,
it can return a cfi_stubs if the atype belongs to a struct_ops.

Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>
Signed-off-by: Amery Hung <ameryhung@gmail.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Reviewed-by: Emil Tsalapatis <emil@etsalapatis.com>
Link: https://patch.msgid.link/20260917200542.3689605-9-ameryhung@gmail.com

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