[PATCH RFCv4] new ioctl TREE_SEARCH_V2

[Gerhard Heift <gerhard@heift.name>]

This patch series first rewrites tree_search to copy found items directly to
userspace and then adds a new ioctl TREE_SEARCH_V2 with which we could store
them in a varying buffer. Now even items larger than 3992 bytes or a large
amount of items can be returned. This is the case 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

RFCv4
  * fixed copy to userspace (used wrong pointers)
  * eliminate a redundant check for nr_items

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