ntb: iounmap MW reg and vbase in error path

The MW regbase and vbase(s) were not being freed if an error occurred
in the vbase allocation loop.  This is corrected by updating the error
path for the allocation loop to err4.

Reported-by: Julia Lawall <julia.lawall@lip6.fr>
Signed-off-by: Jon Mason <jdmason@kudzu.us>