Add compat.h include to avoid a potential build issue:

io_uring/register.c:281:6: error: call to undeclared function 'in_compat_syscall'; ISO C99 and later do not support implicit function declarations [-Werror,-Wimplicit-function-declaration]

if (in_compat_syscall()) {
    ^
1 warning generated.
io_uring/register.c:282:9: error: call to undeclared function 'compat_get_bitmap'; ISO C99 and later do not support implicit function declarations [-Werror,-Wimplicit-function-declaration]
ret = compat_get_bitmap(cpumask_bits(new_mask),
      ^

Fixes: c43203154d8a ("io_uring/register: move io_uring_register(2) related code to register.c")
Reported-by: Manu Bretelle <chantra@meta.com>
Reviewed-by: Jeff Moyer <jmoyer@redhat.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

The tail of the provided ring buffer is shared between the kernel and
the application, but the head is private to the kernel as the
application doesn't need to see it. However, this also prevents the
application from knowing how many buffers the kernel has consumed.
Usually this is fine, as the information is inherently racy in that
the kernel could be consuming buffers continually, but for cleanup
purposes it may be relevant to know how many buffers are still left
in the ring.

Add IORING_REGISTER_PBUF_STATUS which will return status for a given
provided buffer ring. Right now it just returns the head, but space
is reserved for more information later in, if needed.

Link: https://github.com/axboe/liburing/discussions/1020
Signed-off-by: Jens Axboe <axboe@kernel.dk>

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>