Allow loading the same DMA address multiple times without any prior unload. The fixes a panic when running rping from userspace.
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()
While at it simplify the use of mutexes protecting the DMA load process.
Sponsored by: Mellanox Technologies