iommu/arm-smmu-v3: Use the new rb tree helpers

iommu/arm-smmu-v3: Use the new rb tree helpers

Since v5.12 the rbtree has gained some simplifying helpers aimed at making
rb tree users write less convoluted boiler plate code. Instead the caller
provides a single comparison function and the helpers generate the prior
open-coded stuff.

Update smmu->streams to use rb_find_add() and rb_find().

Tested-by: Nicolin Chen <nicolinc@nvidia.com>
Reviewed-by: Mostafa Saleh <smostafa@google.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>
Link: https://lore.kernel.org/r/1-v3-9fef8cdc2ff6+150d1-smmuv3_tidy_jgg@nvidia.com
Signed-off-by: Will Deacon <will@kernel.org>