eal/linux: fix parsing zero socket memory and limits

eal/linux: fix parsing zero socket memory and limits

Modern memory mode allowes to not reserve any memory by the
'--socket-mem' option. i.e. it could be possible to specify
zero preallocated memory like '--socket-mem 0'.
Also, it should be possible to configure unlimited memory
allocations by '--socket-limit 0'.

Both cases are impossible now and blocks starting the DPDK
application:

    ./dpdk-app --socket-limit 0 <...>
    EAL: invalid parameters for --socket-limit
    EAL: Invalid 'command line' arguments.
    Unable to initialize DPDK: Invalid argument

Fixes: 6b42f75632f0 ("eal: enable non-legacy memory mode")
Cc: stable@dpdk.org
Signed-off-by: Ilya Maximets <i.maximets@samsung.com>
Acked-by: Anatoly Burakov <anatoly.burakov@intel.com>