Fix regression issues after r346645 in the LinuxKPI
1) Allow loading the same DMA address multiple times without any prior unload.
2) SG list must be mapped AS-IS without any optimisations.
3) sg_dma_len() must be equal to sg->length. Many drivers assume this.
panic: pctrie_insert: key is already present
panic()
pctrie_insert()
linux_dma_map_sg_attrs()
ib_umem_get()
mr_umem_get()
mlx5_ib_reg_user_mr()
ib_uverbs_reg_mr()
ib_uverbs_write()
linux_file_write()
dofilewrite()
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