Fix decoding of struct msghdr.msg_name* arguments of recvmsg syscall

Fix decoding of struct msghdr.msg_name* arguments of recvmsg syscall

As the msghdr.msg_namelen argument of recvmsg syscall has the same
read-write semantics as the address length argument of recvfrom syscall,
parser of recvmsg syscall needs a similar fix.

* defs.h (fetch_msghdr_namelen): New prototype.
(decode_msghdr): Add "int *" argument.
* msghdr.c (print_msghdr): Likewise.  Treat it as a pointer to struct
msghdr.msg_namelen passed to the kernel.  Pass to decode_sockaddr
the size of socket address actually returned by the kernel.
Print both user and kernel address lengths when the value changes.
(decode_msghdr, decode_mmsghdr): Add "int *" argument,
forward it to print_msghdr.
(decode_mmsgvec): Update decode_mmsghdr invocation.
(fetch_msghdr_namelen): New function.
* net.c (SYS_FUNC(sendmsg)): Update decode_msghdr invocation.
(SYS_FUNC(recvmsg)): Use fetch_msghdr_namelen on entering to save
struct msghdr.msg_namelen.  On exiting, pass the saved value
to decode_msghdr.