Add a enum for decoding to tprint_iov() and tprint_iov_upto()

Add a enum for decoding to tprint_iov() and tprint_iov_upto()

Introduce a new type iov_decode which will be used instead of the integer
"decode" as a parameter.

* defs.h (iov_decode): New enum.
(tprint_iov, tprint_iov_upto): Change type of "decode_iov" to enum iov_decode.
* aio.c (print_iocb): Change type of "decode_iov" to enum iov_decode in
tprint_iov() call.
* keyctl.c (keyctl_instantiate_key_iov): Likewise.
* process.c (ptrace): Likewise.
* process_vm.c (process_vm_readv, process_vm_writev): Likewise.
* io.c (writev, do_pwritev, vmsplice): Likewise.
(print_iovec): Replace the condition with a switch.
(tprint_iov_upto): Change type of "decode_iov" to enum iov_decode.
(readv, do_preadv): Change type of "decode_iov" to enum iov_decode in
tprint_iov_upto() call.
* scsi.c (print_sg_io_v3_req, print_sg_io_v3_res, print_sg_io_v4_req,
print_sg_io_v4_res): Likewise.
* net.c (do_msghdr): Adapt call of tprint_iov_upto().