LinuxKPI: Revert MFC of r347892

Revert until we can figure out how to keep compatibility with drm-kmod
ports between 12.0 and 12-STABLE/12.1.

MFC r347892:
LinuxKPI: Finalize import of seq_file.

seq_file.h and linux_seq_file.c was imported form ports earlier but
linux_seq_file.c was never compiled in with the module. With this
commit base

MFC r347892:
LinuxKPI: Finalize import of seq_file.

seq_file.h and linux_seq_file.c was imported form ports earlier but
linux_seq_file.c was never compiled in with the module. With this
commit base seq_file will replace ports seq_file and it required a
few modifications to not break functionality and build.

Reviewed by: hps
Approved by: imp (mentor), hps

show more ...

MFC r344485:
import linux debugfs support

Reviewed by: hps@
Sponsored by: iX Systems
Differential Revision: https://reviews.freebsd.org/D19258