linuxkpi: Define `seq_has_overflowed()` and `single_open_size()`

[dumbbell]

Summary

This required non-trivial changes to linux_seq_file.c to manage a new (struct seq_file)->size field. This field is read directly by DRM drivers, so we can't alias it to a call to sbuf_len(9).

single_open_size() also depended on the ability to allocate the sbuf with a specified size instead of relying on sbuf_new_auto().

This is part of the update of the DRM drivers to Linux 5.17.