io_uring/rsrc: add huge page accounting for registered buffers

Track huge page references in a per-ring xarray to prevent double
accounting when the same huge page is used by multiple registered
buffers, either within the same ring or across cloned rings.

When registering buffers backed by huge pages, we need to account for
RLIMIT_MEMLOCK. But if multiple buffers share the same huge page (common
with cloned buffers), we must not account for the same page multiple
times. Similarly, we must only unaccount when the last reference to a
huge page is released.

Maintain a per-ring xarray (hpage_acct) that tracks reference counts for
each huge page. When registering a buffer, for each unique huge page,
increment it's accounting reference count, and only account pages that
are newly added.

When unregistering a buffer, for each unique huge page, decrement its
refcount. Once the refcount hits zero, the page is unaccounted.

Note: any account is done against the ctx->user that was assigned when
the ring was setup. As before, if root is running the operation, no
accounting is done.

With these changes, any use of imu->acct_pages is also dead, hence kill
it from struct io_mapped_ubuf. This shrinks it from 54b to 48b on a
64-bit arch.

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