Btrfs: fix a crash when running balance and defrag concurrently

Running balance and defrag concurrently can end up with a crash:

kernel BUG at fs/btrfs/relocation.c:4528!
RIP: 0010:[<ffffffffa01ac33b>]  [<ffffffffa01ac33b>] btrfs_reloc_cow_block+ 0x1eb/0x230 [btrfs]
Call Trace:
  [<ffffffffa01398c1>] ? update_ref_for_cow+0x241/0x380 [btrfs]
  [<ffffffffa0180bad>] ? copy_extent_buffer+0xad/0x110 [btrfs]
  [<ffffffffa0139da1>] __btrfs_cow_block+0x3a1/0x520 [btrfs]
  [<ffffffffa013a0b6>] btrfs_cow_block+0x116/0x1b0 [btrfs]
  [<ffffffffa013ddad>] btrfs_search_slot+0x43d/0x970 [btrfs]
  [<ffffffffa0153c57>] btrfs_lookup_file_extent+0x37/0x40 [btrfs]
  [<ffffffffa0172a5e>] __btrfs_drop_extents+0x11e/0xae0 [btrfs]
  [<ffffffffa013b3fd>] ? generic_bin_search.constprop.39+0x8d/0x1a0 [btrfs]
  [<ffffffff8117d14a>] ? kmem_cache_alloc+0x1da/0x200
  [<ffffffffa0138e7a>] ? btrfs_alloc_path+0x1a/0x20 [btrfs]
  [<ffffffffa0173ef0>] btrfs_drop_extents+0x60/0x90 [btrfs]
  [<ffffffffa016b24d>] relink_extent_backref+0x2ed/0x780 [btrfs]
  [<ffffffffa0162fe0>] ? btrfs_submit_bio_hook+0x1e0/0x1e0 [btrfs]
  [<ffffffffa01b8ed7>] ? iterate_inodes_from_logical+0x87/0xa0 [btrfs]
  [<ffffffffa016b909>] btrfs_finish_ordered_io+0x229/0xac0 [btrfs]
  [<ffffffffa016c3b5>] finish_ordered_fn+0x15/0x20 [btrfs]
  [<ffffffffa018cbe5>] worker_loop+0x125/0x4e0 [btrfs]
  [<ffffffffa018cac0>] ? btrfs_queue_worker+0x300/0x300 [btrfs]
  [<ffffffff81075ea0>] kthread+0xc0/0xd0
  [<ffffffff81075de0>] ? insert_kthread_work+0x40/0x40
  [<ffffffff8164796c>] ret_from_fork+0x7c/0xb0
  [<ffffffff81075de0>] ? insert_kthread_work+0x40/0x40
----------------------------------------------------------------------

It turns out to be that balance operation will bump root's @last_snapshot,
which enables snapshot-aware defrag path, and backref walking stuff will
find data reloc tree as refs' parent, and hit the BUG_ON() during COW.

As data reloc tree's data is just for relocation purpose, and will be deleted right
after relocation is done, it's unnecessary to walk those refs belonged to data reloc
tree, it'd be better to skip them.

Signed-off-by: Liu Bo <bo.li.liu@oracle.com>
Signed-off-by: Josef Bacik <jbacik@fusionio.com>
Signed-off-by: Chris Mason <chris.mason@fusionio.com>