cgroup: mark cgroup_get() with __maybe_unused

cgroup: mark cgroup_get() with __maybe_unused

a590b90d472f ("cgroup: fix spurious warnings on cgroup_is_dead() from
cgroup_sk_alloc()") converted most cgroup_get() usages to
cgroup_get_live() leaving cgroup_sk_alloc() the sole user of
cgroup_get().  When !CONFIG_SOCK_CGROUP_DATA, this ends up triggering
unused warning for cgroup_get().

Silence the warning by adding __maybe_unused to cgroup_get().

Reported-by: Stephen Rothwell <[email protected]>
Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Tejun Heo <[email protected]>