mem: check for contiguousness in external segments

mem: check for contiguousness in external segments

For IOVA as VA mode, we assume that memory is contiguous. However,
for external segments that assumption may not necessarily hold.
Fix the code to not assume that external memory segments are
contiguous even in IOVA as VA mode.

Fixes: 5282bb1c3695 ("mem: allow memseg lists to be marked as external")

Signed-off-by: Anatoly Burakov <anatoly.burakov@intel.com>