ASoC: SOF: ipc3-dtrace: uninitialized data in dfsentry_trace_filter_write()

ASoC: SOF: ipc3-dtrace: uninitialized data in dfsentry_trace_filter_write()

This doesn't check how many bytes the simple_write_to_buffer() writes to
the buffer.  The only thing that we know is that the first byte is
initialized and the last byte of the buffer is set to NUL.  However
the middle bytes could be uninitialized.

There is no need to use simple_write_to_buffer().  This code does not
support partial writes but instead passes "pos = 0" as the starting
offset regardless of what the user passed as "*ppos".  Just use the
copy_from_user() function and initialize the whole buffer.

Fixes: 671e0b90051e ("ASoC: SOF: Clone the trace code to ipc3-dtrace as fw_tracing implementation")
Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
Link: https://lore.kernel.org/r/74148292-ce4d-4e01-a1a7-921e6767da14@moroto.mountain
Signed-off-by: Mark Brown <broonie@kernel.org>