i40e: remove read access to debugfs files

In the Linux kernel, the following vulnerability has been resolved:

i40e: remove read access to debugfs files

The 'command' and 'netdev_ops' debugfs files are a legacy debugging
interface supported by the i40e driver since its early days by commit
02e9c290814c ("i40e: debugfs interface").

Both of these debugfs files provide a read handler which is mostly useless,
and which is implemented with questionable logic. They both use a static
256 byte buffer which is initialized to the empty string. In the case of
the 'command' file this buffer is literally never used and simply wastes
space. In the case of the 'netdev_ops' file, the last command written is
saved here.

On read, the files contents are presented as the name of the device
followed by a colon and then the contents of their respective static
buffer. For 'command' this will always be "<device>: ". For 'netdev_ops',
this will be "<device>: <last command written>". But note the buffer is
shared between all devices operated by this module. At best, it is mostly
meaningless information, and at worse it could be accessed simultaneously
as there doesn't appear to be any locking mechanism.

We have also recently received multiple reports for both read functions
about their use of snprintf and potential overflow that could result in
reading arbitrary kernel memory. For the 'command' file, this is definitely
impossible, since the static buffer is always zero and never written to.
For the 'netdev_ops' file, it does appear to be possible, if the user
carefully crafts the command input, it will be copied into the buffer,
which could be large enough to cause snprintf to truncate, which then
causes the copy_to_user to read beyond the length of the buffer allocated
by kzalloc.

A minimal fix would be to replace snprintf() with scnprintf() which would
cap the return to the number of bytes written, preventing an overflow. A
more involved fix would be to drop the mostly useless static buffers,
saving 512 bytes and modifying the read functions to stop needing those as
input.

Instead, lets just completely drop the read access to these files. These
are debug interfaces exposed as part of debugfs, and I don't believe that
dropping read access will break any script, as the provided output is
pretty useless. You can find the netdev name through other more standard
interfaces, and the 'netdev_ops' interface can easily result in garbage if
you issue simultaneous writes to multiple devices at once.

In order to properly remove the i40e_dbg_netdev_ops_buf, we need to
refactor its write function to avoid using the static buffer. Instead, use
the same logic as the i40e_dbg_command_write, with an allocated buffer.
Update the code to use this instead of the static buffer, and ensure we
free the buffer on exit. This fixes simultaneous writes to 'netdev_ops' on
multiple devices, and allows us to remove the now unused static buffer
along with removing the read access.