mem: warn once if pagemap is unreadable

mem: warn once if pagemap is unreadable

Newer kernels make this unreadable for security reasons for non-roots.
Running the application will then fill the logs with

  rte_mem_virt2phy: cannot open /proc/self/pagemap

messages.

However, there are cases when DPDK is and should be run as non-root,
without the need for virtual-to-physical address translations: a
typical example is when working with PCAP input/output. This patch
adds a start-time check for /proc/self/pagemap readability, and
directly returns an error code from rte_mem_virt2phy().

This way, there is only a one-time warning at startup instead of
constant warnings all the time.

Signed-off-by: Simon Kagstrom <simon.kagstrom@netinsight.net>
Signed-off-by: Johan Faltstrom <johan.faltstrom@netinsight.net>