vc_screen: extract vcs_read_buf_header

The attribute header handling is terrible in vcs_read_buf. Separate it
to a new function and simply do memmove (of up to 4 bytes) to the start
of the con_buf -- if user seeked.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Link: https://lore.kernel.org/r/20200818085706.12163-15-jslaby@suse.cz


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>