untangling ep_call_nested(): it's all serialized on epmutex.

IOW,
	* no locking is needed to protect the list
	* the list is actually a stack
	* no need to check ->ctx
	* it can bloody well be a static 5-element array - nobody is
going to be accessing it in parallel.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>