I've rewritten the single copy pipe code again.
Main changes:

* doesn't use map_user_kiobuf anymore. That function locked the pages
into memory. DoS attacks were possible. Now pages stay pageable.
* simpler code, fewer loops.
* support added for set_fs(KERNEL_DS)+pipe_write
* all transfers that block now use single copy. This should reduce the
number of context switches.

The patch is not yet fully tested, but boots, compiles kernels and
survives man <long man page>.

Should I merge pio_copy_to_user() with access_process_vm()?
The main part of pio_copy_to_user() is access_process_vm(), but with
copy_to_user() instead of memmove().

--
	Manfred