datagram: fix kernel-doc comments

An underscore in the kernel-doc comment section has special meaning
and mis-use generates an errors.

./net/core/datagram.c:207: ERROR: Unknown target name: "msg".
./net/core/datagram.c:379: ERROR: Unknown target name: "msg".
./net/core/datagram.c:816: ERROR: Unknown target name: "t".

Signed-off-by: Stephen Hemminger <sthemmin@microsoft.com>
Signed-off-by: David S. Miller <davem@davemloft.net>