NET: NETROM: When adding a route verify length of mnemonic string.

struct nr_route_struct's mnemonic permits a string of up to 7 bytes to be
used.  If userland passes a not zero terminated string to the kernel adding
a node to the routing table might result in the kernel attempting to read
copy a too long string.

Mnemonic is part of the NET/ROM routing protocol; NET/ROM routing table
updates only broadcast 6 bytes.  The 7th byte in the mnemonic array exists
only as a \0 termination character for the kernel code's convenience.

Fixed by rejecting mnemonic strings that have no terminating \0 in the first
7 characters.  Do this test only NETROM_NODE to avoid breaking NETROM_NEIGH
where userland might passing an uninitialized mnemonic field.

Initial patch by Dan Carpenter <dan.carpenter@oracle.com>.

Signed-off-by: Ralf Baechle <ralf@linux-mips.org>
Cc: Dan Carpenter <dan.carpenter@oracle.com>
Cc: Walter Harms <wharms@bfs.de>
Cc: Thomas Osterried <thomas@osterried.de>
Acked-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>