[PATCH RFCv3] new ioctl TREE_SEARCH_V2

[Gerhard Heift <gerhard@heift.name>]

This patch series adds a new ioctl TREE_SEARCH_V2 with which we could store the
results in a varying buffer. With that even items larger than 3992 bytes or a
large amount of items can be returned. This is the case e.g. for some
EXTENT_CSUM items, which could have a size up to 16k.

I had a few questions:

  Which value should I assign to TREE_SEARCH_V2?
  * Chosen value is ok [1].

  Should we limit the buffer size?
  * David suggested [1] a minimum of 64k, I've chosen a cap of 16M.

  What about documentation?
  * I documented the new struct in the header file.

Gerhard

[1] http://article.gmane.org/gmane.comp.file-systems.btrfs/32060

Changelog

RFCv3
  * introduced read_extent_buffer_to_user
  * direct copy to user without intermediate buffer
  * use local variables for args
  * fixed wrong error code
  * removed unused var check
  * fixed minor style issues
  * return needed buffer to userspace on EOVERFLOW

RFCv2
  * fixed a build bug caused by using a wrong patch
  * added a patch to expand a buffer lifetime