Btrfs: add a reschedule point in btrfs_find_all_roots()

I can easily trigger the following warnings when enabling quota
in my virtual machine(running Opensuse), Steps are firstly creating
a subvolume full of fragment extents, and then create many snapshots
(500 in my test case).

[ 2362.808459] BUG: soft lockup - CPU#0 stuck for 22s! [btrfs-qgroup-re:1970]

[ 2362.809023] task: e4af8450 ti: e371c000 task.ti: e371c000
[ 2362.809026] EIP: 0060:[<fa38f4ae>] EFLAGS: 00000246 CPU: 0
[ 2362.809049] EIP is at __merge_refs+0x5e/0x100 [btrfs]
[ 2362.809051] EAX: 00000000 EBX: cfadbcf0 ECX: 00000000 EDX: cfadbcb0
[ 2362.809052] ESI: dd8d3370 EDI: e371dde0 EBP: e371dd6c ESP: e371dd5c
[ 2362.809054]  DS: 007b ES: 007b FS: 00d8 GS: 00e0 SS: 0068
[ 2362.809055] CR0: 80050033 CR2: ac454d50 CR3: 009a9000 CR4: 001407d0
[ 2362.809099] Stack:
[ 2362.809100]  00000001 e371dde0 dfcc6890 f29f8000 e371de28 fa39016d 00000011 00000001
[ 2362.809105]  99bfc000 00000000 93928000 00000000 00000001 00000050 e371dda8 00000001
[ 2362.809109]  f3a31000 f3413000 00000001 e371ddb8 000040a8 00000202 00000000 00000023
[ 2362.809113] Call Trace:
[ 2362.809136]  [<fa39016d>] find_parent_nodes+0x34d/0x1280 [btrfs]
[ 2362.809156]  [<fa391172>] btrfs_find_all_roots+0xb2/0x110 [btrfs]
[ 2362.809174]  [<fa3934a8>] btrfs_qgroup_rescan_worker+0x358/0x7a0 [btrfs]
[ 2362.809180]  [<c024d0ce>] ? lock_timer_base.isra.39+0x1e/0x40
[ 2362.809199]  [<fa3648df>] worker_loop+0xff/0x470 [btrfs]
[ 2362.809204]  [<c027a88a>] ? __wake_up_locked+0x1a/0x20
[ 2362.809221]  [<fa3647e0>] ? btrfs_queue_worker+0x2b0/0x2b0 [btrfs]
[ 2362.809225]  [<c025ebbc>] kthread+0x9c/0xb0
[ 2362.809229]  [<c06b487b>] ret_from_kernel_thread+0x1b/0x30
[ 2362.809233]  [<c025eb20>] ? kthread_create_on_node+0x110/0x110

By adding a reschedule point at the end of btrfs_find_all_roots(), i no longer
hit these warnings.

Cc: Josef Bacik <jbacik@fb.com>
Signed-off-by: Wang Shilong <wangsl.fnst@cn.fujitsu.com>
Reviewed-by: David Sterba <dsterba@suse.cz>
Signed-off-by: Josef Bacik <jbacik@fb.com>
Signed-off-by: Chris Mason <clm@fb.com>