io_uring: add wrappers for memory accounting

Facilitate separation of locked memory usage reporting vs. limiting for
upcoming patches.  No functional changes.

Signed-off-by: Bijan Mottahedeh <bijan.mottahedeh@oracle.com>
[axboe: kill unnecessary () around return in io_account_mem()]
Signed-off-by: Jens Axboe <axboe@kernel.dk>