Btrfs: Switch the extent buffer rbtree into a radix tree

This patch reduces the CPU time spent in the extent buffer search by using the
radix tree instead of the rbtree and using the rcu lock instead of the spin
lock.

I did a quick test by the benchmark tool[1] and found the patch improve the
file creation/deletion performance problem that I have reported[2].

Before applying this patch:
Create files:
	Total files: 50000
	Total time: 0.971531
	Average time: 0.000019
Delete files:
	Total files: 50000
	Total time: 1.366761
	Average time: 0.000027

After applying this patch:
Create files:
	Total files: 50000
	Total time: 0.927455
	Average time: 0.000019
Delete files:
	Total files: 50000
	Total time: 1.292280
	Average time: 0.000026

[1] http://marc.info/?l=linux-btrfs&m=128212635122920&q=p3
[2] http://marc.info/?l=linux-btrfs&m=128212635122920&w=2

Signed-off-by: Miao Xie <miaox@cn.fujitsu.com>
Signed-off-by: Chris Mason <chris.mason@oracle.com>