cgroup: Move cgroup_parse_float() implementation out of CONFIG_SYSFS

a5e112e6 ("cgroup: add cgroup_parse_float()") accidentally added
cgroup_parse_float() inside CONFIG_SYSFS block.  Move it outside so
that it doesn't cause failures on !CONFIG_SYSFS builds.

Signed-off-by: Tejun Heo <tj@kernel.org>
Fixes: a5e112e6 ("cgroup: add cgroup_parse_float()")