tap: fix tap interface not working on Arm issue

tap: fix tap interface not working on Arm issue

The VPP code tries to set all userspace memory in the table via IOCTL
to VHOST_SET_MEM_TABLE. But on aarch64, the userspace address range is
larger (48 bits) than that on x86 (47 bits). Below is an segment from
/proc/[vpp]/maps.

fffb41200000-fffb43a00000 rw-s 00000000 00:0e 532232
/anon_hugepage (deleted)

Instead of setting all userspace memory space to vhost-net, will only set
the address space reserved by pmalloc module during initialization.

Type: fix

Change-Id: I91cb35e990869b42094cf2cd0512593733d33677
Signed-off-by: Lijian Zhang <Lijian.Zhang@arm.com>
Reviewed-by: Steve Capper <Steve.Capper@arm.com>