FROMLIST: iommu/io-pgtable: Introduce map_sg() as a page table op

While mapping a scatter-gather list, iommu_map_sg() calls
into the IOMMU driver through an indirect call, which can
call into the io-pgtable code through another indirect call.

This sequence of going through the IOMMU core code, the IOMMU
driver, and finally the io-pgtable code, occurs for every
element in the scatter-gather list, in the worse case, which
is not optimal.

Introduce a map_sg callback in the io-pgtable ops so that
IOMMU drivers can invoke it with the complete scatter-gather
list, so that it can be processed within the io-pgtable
code entirely, reducing the number of indirect calls, and
boosting overall iommu_map_sg() performance.

Bug: 176779203
Link: https://lore.kernel.org/linux-iommu/1610376862-927-1-git-send-email-isaacm@codeaurora.org/T/#t


Change-Id: I4b2088dd08eb97dcd94a6c6968082a3c4395351a
Signed-off-by: Isaac J. Manjarres <isaacm@codeaurora.org>
Tested-by: Sai Prakash Ranjan <saiprakash.ranjan@codeaurora.org>