io_uring/rsrc: use kvfree() for the imu cache

io_uring/rsrc: use kvfree() for the imu cache

Currently anything that requires kvmalloc_flex() for allocations will
not get re-cached, and hence the cache freeing path is correct in that
it always uses kfree() to free the allocated memory. But this seems a
bit fragile as it's something that could get mix should that situation
change, so switch io_free_imu() and io_alloc_cache_free() to use kvfree
as the desctructor.

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