iio: buffer-dma: split iio_dma_buffer_fileio_free() function

This change splits the logic into a separate function, which will be
re-used later.

Signed-off-by: Alexandru Ardelean <alexandru.ardelean@analog.com>
Cc: Alexandru Ardelean <ardeleanalex@gmail.com>
Signed-off-by: Paul Cercueil <paul@crapouillou.net>
Link: https://lore.kernel.org/r/20231219175009.65482-3-paul@crapouillou.net


Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>