fbdev: Return number of bytes read or written

Always return the number of bytes read or written within the
framebuffer. Only return an errno code if framebuffer memory
was not touched. This is the semantics required by POSIX and
makes fb_read() and fb_write() compatible with IGT tests. [1]

This bug has been fixed for fb_write() long ago by
commit 6a2a8866 ("[PATCH] fbdev: Fix return error of
fb_write"). The code in fb_read() and the corresponding fb_sys_()
helpers was forgotten.

It can happen that copy_{from, to}_user() only partially copies
the given buffer. Take this into account when calculating the
number of bytes.

v2:
	* consider return value from copy_{from,to}_user() (Geert)

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Tested-by: Sui Jingfeng <suijingfeng@loongson.cn>
Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Acked-by: Helge Deller <deller@gmx.de>
Link: https://gitlab.freedesktop.org/drm/igt-gpu-tools/-/blob/master/tests/fbdev.c # 1
Link: https://patchwork.freedesktop.org/patch/msgid/20230428122452.4856-15-tzimmermann@suse.de