ethtool: Null-terminate filename passed to ethtool_ops::flash_device

ethtool: Null-terminate filename passed to ethtool_ops::flash_device

commit 786f528119722f564a22ad953411374e06116333 upstream.

The parameters for ETHTOOL_FLASHDEV include a filename, which ought to
be null-terminated.  Currently the only driver that implements
ethtool_ops::flash_device attempts to add a null terminator if
necessary, but does it wrongly.  Do it in the ethtool core instead.

Signed-off-by: Ben Hutchings <bhutchings@solarflare.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>