debugfs: add small file operations for most files

debugfs: add small file operations for most files

As struct file_operations is really big, but (most) debugfs
files only use simple_open, read, write and perhaps seek, and
don't need anything else, this wastes a lot of space for NULL
pointers.

Add a struct debugfs_short_fops and some bookkeeping code in
debugfs so that users can use that with debugfs_create_file()
using _Generic to figure out which function to use.

Converting mac80211 to use it where possible saves quite a
bit of space:

1010127  205064    1220 1216411  128f9b net/mac80211/mac80211.ko (before)
 981199  205064    1220 1187483  121e9b net/mac80211/mac80211.ko (after)
-------
 -28928 = ~28KiB

With a marginal space cost in debugfs:

   8701	    550	     16	   9267	   2433	fs/debugfs/inode.o (before)
  25233	    325	     32	  25590	   63f6	fs/debugfs/file.o  (before)
   8914	    558	     16	   9488	   2510	fs/debugfs/inode.o (after)
  25380	    325	     32	  25737	   6489	fs/debugfs/file.o  (after)
---------------
   +360      +8

(All on x86-64)

A simple spatch suggests there are more than 300 instances,
not even counting the ones hidden in macros like in mac80211,
that could be trivially converted, for additional savings of
about 240 bytes for each.

Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Link: https://patch.msgid.link/20241022151838.26f9925fb959.Ia80b55e934bbfc45ce0df42a3233d34b35508046@changeid
Signed-off-by: Johannes Berg <johannes.berg@intel.com>