linuxkpi: Define seq_has_overflowed() and single_open_size()

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().

Reviewed by: manu
Approved by: manu
Differential Revision: https://reviews.freebsd.org/D39056