mem: fix check of physical address retrieval

mem: fix check of physical address retrieval

In rte_mem_virt2phy: Value returned from a function and indicating the
number of bytes was ignored. This could cause a wrong pfn (page frame
number) mask read from pagemap file.
When read returns less than the number of sizeof(uint64_t) bytes,
function rte_mem_virt2phy returns error.

Coverity issue: 13212
Fixes: 40b966a211ab ("ivshmem: library changes for mmaping using ivshmem")
Signed-off-by: Michal Jastrzebski <michalx.k.jastrzebski@intel.com>