bpf: Revert "bpf: drop KF_ACQUIRE flag on BPF kfunc bpf_get_root_mem_cgroup()"

bpf: Revert "bpf: drop KF_ACQUIRE flag on BPF kfunc bpf_get_root_mem_cgroup()"

This reverts commit e463b6de9da1 ("bpf: drop KF_ACQUIRE flag on BPF
kfunc bpf_get_root_mem_cgroup()").

The original commit removed the KF_ACQUIRE flag from
bpf_get_root_mem_cgroup() under the assumption that it resulted in
simplified usage. This stemmed from the fact that
bpf_get_root_mem_cgroup() inherently returns a reference to an object
which technically isn't reference counted, therefore there is no
strong requirement to call a matching bpf_put_mem_cgroup() on the
returned reference.

Although technically correct, as per the arguments in the thread [0],
dropping the KF_ACQUIRE flag and losing reference tracking semantics
negatively impacted the usability of bpf_get_root_mem_cgroup() in
practice.

[0] https://lore.kernel.org/bpf/878qdx6yut.fsf@linux.dev/

Link: https://lore.kernel.org/bpf/CAADnVQ+6d1Lj4dteAv8u62d7kj3Ze5io6bqM0xeQd-UPk9ZgJQ@mail.gmail.com/
Signed-off-by: Matt Bobrowski <mattbobrowski@google.com>
Link: https://lore.kernel.org/r/20260121090001.240166-1-mattbobrowski@google.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>