selftests/net: tcp_mmap: fix clang warning for target arch PowerPC

When size_t maps to unsigned int (e.g. on 32-bit powerpc), then the
comparison with 1<<35 is always true. Clang 9 threw:
warning: result of comparison of constant 34359738368 with \
expression of type 'size_t' (aka 'unsigned int') is always true \
[-Wtautological-constant-out-of-range-compare]
        while (total < FILE_SZ) {

Tested: make -C tools/testing/selftests TARGETS="net" run_tests

Fixes: 192dc405

 ("selftests: net: add tcp_mmap program")
Signed-off-by: Tanner Love <tannerlove@google.com>
Acked-by: Willem de Bruijn <willemb@google.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>