fix /proc/vmcore seek

[Andrew Morton]

On Thu, 2 Sep 2010 14:20:19 +0200
Arnd Bergmann <arnd at arndb.de> wrote:

> Commit 73296bc611 "procfs: Use generic_file_llseek in /proc/vmcore" broke
> seeking on /proc/vmcore. This changes it back to use default_llseek
> in order to restore the original behaviour.

"broke" isn't a useful description of a bug :(

A bit of list-trolling indicates that this change caused makedumpfile
to hang during kexec dumping, yes?  If so, why did this happen?  How
did the switch to generic_file_llseek() cause the /proc/vmcore seek
behaviour to change?

I'm not seeing a lot of difference between default_llseek() and
generic_file_llseek() apart from file->f_mapping->host versus
file->f_path.dentry->d_inode.  Was it that, or what?

etcetera.  Better patch descriptions, please!