cgroup: Restructure release_agent_path handling

cgrp->root->release_agent_path is protected by both cgroup_mutex and
release_agent_path_lock and readers can hold either one. The
dual-locking scheme was introduced while breaking a locking dependency
issue around cgroup_mutex but doesn't make sense anymore given that
the only remaining reader which uses cgroup_mutex is
cgroup1_releaes_agent().

This patch updates cgroup1_release_agent() to use
release_agent_path_lock so that release_agent_path is always protected
only by release_agent_path_lock.

While at it, convert strlen() based empty string checks to direct
tests on the first character as suggested by Linus.

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>