vfio: skip DMA map failure if already mapped

vfio: skip DMA map failure if already mapped

Currently vfio DMA map function will fail in case the same memory
segment is mapped twice.

This is too strict, as this is not an error to map the same memory
twice.

Instead, use the kernel return value to detect such state and have the
DMA function to return as successful.

For type1 mapping the kernel driver returns EEXISTS.
For spapr mapping EBUSY is returned since kernel 4.10.

Signed-off-by: Shahaf Shuler <shahafs@mellanox.com>
Acked-by: Anatoly Burakov <anatoly.burakov@intel.com>
Acked-by: Gaetan Rivet <gaetan.rivet@6wind.com>