eventpoll: pass struct epoll_filefd through ep_find() and ep_insert()

eventpoll: pass struct epoll_filefd through ep_find() and ep_insert()

Have ep_find() and ep_insert() take a struct epoll_filefd rather
than a file/fd tuple. Kill off ep_set_ffd() as it's now no longer
needed.

No functional change. This is a prep patch for adding a file based
do_epoll_ctl() variant.

Signed-off-by: Jens Axboe <axboe@kernel.dk>
Link: https://patch.msgid.link/20260514140817.623026-2-axboe@kernel.dk
Reviewed-by: Christian Brauner <brauner@kernel.org>
Signed-off-by: Christian Brauner <brauner@kernel.org>