io_uring/register: move io_uring_register(2) related code to register.c

Most of this code is basically self contained, move it out of the core
io_uring file to bring a bit more separation to the registration related
bits. This moves another ~10% of the code into register.c.

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

io_uring/Makefile

@@ -8,6 +8,6 @@ obj-$(CONFIG_IO_URING)		+= io_uring.o xattr.o nop.o fs.o splice.o \
 					statx.o net.o msg_ring.o timeout.o \
 					sqpoll.o fdinfo.o tctx.o poll.o \
 					cancel.o kbuf.o rsrc.o rw.o opdef.o \
-					notif.o waitid.o
+					notif.o waitid.o register.o
 obj-$(CONFIG_IO_WQ)		+= io-wq.o
 obj-$(CONFIG_FUTEX)		+= futex.o