dma-buf: update exp_name when using dma_buf_export()

commit c0b00a5 ("dma-buf: update debugfs output") modified the
default exporter name to be the KBUILD_MODNAME pre-processor
macro instead of __FILE__ but the documentation was not updated.

Also the "Supporting existing mmap interfaces in exporters" section
title seems wrong since talks about the interface used by importers.

Signed-off-by: Javier Martinez Canillas <javier.martinez@collabora.co.uk>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>