io_uring/eventfd: move eventfd handling to separate file

This is pretty nicely abstracted already, but let's move it to a separate
file rather than have it in the main io_uring file. With that, we can
also move the io_ev_fd struct and enum out of global scope.

Signed-off-by: Jens Axboe <axboe@kernel.dk>

io_uring/Makefile

@@ -4,9 +4,9 @@
 
 obj-$(CONFIG_IO_URING)		+= io_uring.o opdef.o kbuf.o rsrc.o notif.o \
 					tctx.o filetable.o rw.o net.o poll.o \
-					uring_cmd.o openclose.o sqpoll.o \
-					xattr.o nop.o fs.o splice.o sync.o \
-					msg_ring.o advise.o openclose.o \
+					eventfd.o uring_cmd.o openclose.o \
+					sqpoll.o xattr.o nop.o fs.o splice.o \
+					sync.o msg_ring.o advise.o openclose.o \
 					epoll.o statx.o timeout.o fdinfo.o \
 					cancel.o waitid.o register.o \
 					truncate.o memmap.o