Fix the following out-of-bounds warnings by enclosing some structure
members into new structure objects upiu_req and upiu_rsp:

include/linux/fortify-string.h:20:29: warning: '__builtin_memcpy' offset [29, 48] from the object at 'treq' is out of the bounds of referenced subobject 'req_header' with type 'struct utp_upiu_header' at offset 16 [-Warray-bounds]
include/linux/fortify-string.h:20:29: warning: '__builtin_memcpy' offset [61, 80] from the object at 'treq' is out of the bounds of referenced subobject 'rsp_header' with type 'struct utp_upiu_header' at offset 48 [-Warray-bounds]
arch/m68k/include/asm/string.h:72:25: warning: '__builtin_memcpy' offset [29, 48] from the object at 'treq' is out of the bounds of referenced subobject 'req_header' with type 'struct utp_upiu_header' at offset 16 [-Warray-bounds]
arch/m68k/include/asm/string.h:72:25: warning: '__builtin_memcpy' offset [61, 80] from the object at 'treq' is out of the bounds of referenced subobject 'rsp_header' with type 'struct utp_upiu_header' at offset 48 [-Warray-bounds]

Refactor the code by making it more structured.

The problem is that the original code is trying to copy data into a bunch
of struct members adjacent to each other in a single call to memcpy(). Now
that a new struct _upiu_req_ enclosing all those adjacent members is
introduced, memcpy() doesn't overrun the length of &treq.req_header,
because the address of the new struct object _upiu_req_ is used as the
destination, instead. The same problem is present when memcpy() overruns
the length of the source &treq.rsp_header; in this case the address of the
new struct object _upiu_rsp_ is used, instead.

Also, this helps with the ongoing efforts to enable -Warray-bounds and
avoid confusing the compiler.

Link: https://github.com/KSPP/linux/issues/109
Link: https://lore.kernel.org/lkml/60640558.lsAxiK6otPwTo9rv%25lkp@intel.com/
Link: https://lore.kernel.org/r/20210331224338.GA347171@embeddedorReported-by: kernel test robot <lkp@intel.com>
Reviewed-by: Avri Altman <avri.altman@wdc.com>
Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>
Build-tested-by: kernel test robot <lkp@intel.com>