mempool: fix returned value after counting objects

mempool: fix returned value after counting objects

rte_mempool_xmem_usage()'s return type is ssize_t which has the same
architecture-dependent width as size_t but is signed.

On 64-bit architectures, returning a negative uint32_t value without casting
to ssize_t first does not work as intended, the sign bit is lost and the
returned value is garbage.

This commit fixes an assertion failure in testpmd on 64 bit architectures
when combining --no-huge and --mp-anon outside of Xen Dom0:

 PANIC in mempool_anon_create():
 line 170        assert "elt_num == mp->size" failed

Fixes: 148f963fb532 ("xen: core library changes")

Signed-off-by: Adrien Mazarguil <adrien.mazarguil@6wind.com>
Acked-by: Konstantin Ananyev <konstantin.ananyev@intel.com>