simplify procfs code for seq_file instances V3

From: Christoph Hellwig
Date: Wed May 16 2018 - 05:51:15 EST


We currently have hundreds of proc files that implement plain, read-only
seq_file based interfaces. This series consolidates them using new
procfs helpers that take the seq_operations or simple show callback
directly.

A git tree is available at:

git://git.infradead.org/users/hch/misc.git proc_create.3

Gitweb:

http://git.infradead.org/users/hch/misc.git/shortlog/refs/heads/proc_create.3

Changes since V2:
- use unsigned int for state_size everywhere
- move state_size around in proc_dir_entry to use a struct packing hole
- update SIZEOF_PDE_INLINE_NAME
- added a new proc_pid_ns helper
- improved a few changelogs
- added back a nubus comment
- minor typo fix
- collected various ACKs

Changes since V1:
- open code proc_create_data to avoid setting not fully initialized
entries live
- use unsigned int for state_size
- dropped the s390/cio/blacklist hunk as it has a write method
- dropped the IPMI patch given that IPMI proc support is scheduled for
removal.