igb_uio: fix possible mmap failure with Linux 4.5

igb_uio: fix possible mmap failure with Linux 4.5

mmap the iomem range of the PCI device fails for kernels that
enabled CONFIG_IO_STRICT_DEVMEM option:

EAL: pci_map_resource():
         cannot mmap(39, 0x7f1c51800000, 0x100000, 0x0):
         Invalid argument (0xffffffffffffffff)

CONFIG_IO_STRICT_DEVMEM is introduced in Linux v4.5 and not enabled
by default:
Linux commit: 90a545e restrict /dev/mem to idle io memory ranges

As a workaround igb_uio can stop reserving PCI memory resources, from
kernel point of view iomem region looks like idle and mmap works
again. This matches uio_pci_generic usage.

With this update device iomem range is not protected against any
other kernel drivers or userspace access. But this  shouldn't
be a problem for dpdk usage module since purpose of the igb_uio
module is to provide userspace access.

Fixes: af75078fece3 ("first public release")

Signed-off-by: Ferruh Yigit <ferruh.yigit@intel.com>