wifi: iwlwifi: correctly lookup DMA address in SG table

The code to lookup the scatter gather table entry assumed that it was
possible to use sg_virt() in order to lookup the DMA address in a mapped
scatter gather table. However, this assumption is incorrect as the DMA
mapping code may merge multiple entries into one. In that case, the DMA
address space may have e.g. two consecutive pages which is correctly
represented by the scatter gather list entry, however the virtual
addresses for these two pages may differ and the relationship cannot be
resolved anymore.

Avoid this problem entirely by working with the offset into the mapped
area instead of using virtual addresses. With that we only use the DMA
length and DMA address from the scatter gather list entries. The
underlying DMA/IOMMU code is therefore free to merge two entries into
one even if the virtual addresses space for the area is not continuous.

Fixes: 90db5075 ("wifi: iwlwifi: use already mapped data when TXing an AMSDU")
Reported-by: Chris Bainbridge <chris.bainbridge@gmail.com>
Closes: https://lore.kernel.org/r/ZrNRoEbdkxkKFMBi@debian.local


Signed-off-by: Benjamin Berg <benjamin.berg@intel.com>
Tested-by: Chris Bainbridge <chris.bainbridge@gmail.com>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://patch.msgid.link/20240812110640.460514-1-benjamin@sipsolutions.net