net: explicit cast in L4 checksum

net: explicit cast in L4 checksum

GCC 8.1 warned:

In function 'rte_ipv4_udptcp_cksum':
rte_byteorder.h:51:24: warning: conversion from 'long unsigned int' to
'uint32_t' {aka 'unsigned int'} may change value [-Wconversion]
 #define rte_bswap16(x) ((uint16_t) (__builtin_constant_p(x) ?  \
                        ^
rte_byteorder.h:85:29: note: in expansion of macro 'rte_bswap16'
 #define rte_be_to_cpu_16(x) rte_bswap16(x)
                             ^~~~~~~~~~~
rte_ip.h:321:11: note: in expansion of macro 'rte_be_to_cpu_16'
  l4_len = rte_be_to_cpu_16(ipv4_hdr->total_length) -
           ^~~~~~~~~~~~~~~~

Also with this one, it is a cast that always occurred
and is just being done explicitly, with no changes to
the generated code.

The warning stack is misleading, it points to the last
element in the macro that produced the lhs of the subtraction
above.  But the only "unsigned long int" in the expression is
the result of the sizeof() on the rhs, it promotes the
subtraction result to unsigned long.  So the error actually
relates to the result of the outer subtraction.

The actual error is "you are trying to put an unsigned long
into a uint32_t".  We always did so, the fix is just to inform
the compiler it is intentional with an explicit cast.

Fixes: 6006818cfb ("net: new checksum functions")
Cc: stable@dpdk.org
Signed-off-by: Andy Green <andy@warmcat.com>
Acked-by: Bruce Richardson <bruce.richardson@intel.com>