Btrfs: add a rb_tree to improve performance of ulist search

Walking backref tree and btrfs quota rely on ulist very much.
This patch tries to use rb_tree to speed up search time.

The original code always checks whether an element
exists before adding a new element, however it costs O(n).

I try to add a rb_tree in the ulist,this is only used to speed up
search. I also do some measurements with quota enabled.

fsstress -p 4 -n 10000

Without this path:
real    0m51.058s       2m4.745s        1m28.222s       1m5.137s
user    0m0.035s        0m0.041s        0m0.105s        0m0.100s
sys     0m12.009s       0m11.246s       0m10.901s       0m10.999s       0m11.287s

With this path:
real    0m55.295s       0m50.960s       1m2.214s        0m48.273s
user    0m0.053s        0m0.095s        0m0.135s        0m0.107s
sys     0m7.766s        0m6.013s        0m6.319s        0m6.030s        0m6.532s

After applying the patch,the execute time is down by ~42%.(11.287s->6.532s)

Signed-off-by: Wang Shilong <wangsl-fnst@cn.fujitsu.com>
Reviewed-by: Miao Xie <miaox@cn.fujitsu.com>
Reviewed-by: Jan Schmidt <list.btrfs@jan-o-sch.net>
Signed-off-by: Josef Bacik <jbacik@fusionio.com>