iio: fix potential out-of-bound write

iio: fix potential out-of-bound write

The buffer is set to 20 characters. If a caller write more characters,
count is truncated to the max available space in "simple_write_to_buffer".
To protect from OoB access, check that the input size fit into buffer and
add a zero terminator after copy to the end of the copied data.

Fixes: 6d5dd486c715 iio: core: make use of simple_write_to_buffer()
Signed-off-by: Markus Burri <markus.burri@mt.com>
Link: https://patch.msgid.link/20250508130612.82270-4-markus.burri@mt.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>