epoll: rename global epmutex

As of 4f04cbaf128 ("epoll: use refcount to reduce ep_mutex contention"),
this lock is now specific to nesting cases - inserting an epoll fd onto
another epoll fd.  Rename the lock to be less generic.

Link: https://lkml.kernel.org/r/20230411234159.20421-1-dave@stgolabs.net


Signed-off-by: Davidlohr Bueso <dave@stgolabs.net>
Cc: Paolo Abeni <pabeni@redhat.com>
Cc: Eric Dumazet <edumazet@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>