kernel-doc: fix function-pointer-parameter parsing

When running "make htmldocs" I'm seeing some non-fatal perl errors caused
by trying to parse the callback function definitions in blk-core.c.

The errors are "Use of uninitialized value in concatenation (.)..."
in combination with:
Warning(linux-2.6.25-rc2/block/blk-core.c:1877): No description found for parameter ''

The function pointers are defined without a * i.e.
int (drv_callback)(struct request *)

The compiler is happy with them, but kernel-doc isn't.

This patch teaches create_parameterlist in kernel-doc to parse this type of
function pointer definition, but is it the right way to fix the problem ?
The problem only seems to occur in blk-core.c.

However with the patch applied, kernel-doc finds the correct parameter
description for the callback in blk_end_request_callback, which is doesn't
normally.

I thought it would be a bit odd to change to code to use the more normal
form of function pointers just to get the documentation to work, so I fixed
...