io_uring: use kvfree() in io_sqe_buffer_register()

Use kvfree() to free the pages and vmas, since they are allocated by
kvmalloc_array() in a loop.

Fixes: d4ef6475 ("io_uring: avoid page allocation warnings")
Signed-off-by: Denis Efremov <efremov@linux.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
Cc: stable@vger.kernel.org
Link: https://lore.kernel.org/r/20200605093203.40087-1-efremov@linux.com