simplify procfs code for seq_file instances V3

[Christoph Hellwig]

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.