btrfs: use verbose assertions in backref.c

btrfs: use verbose assertions in backref.c

While debugging a relocation issue I hit an assertion in backref.c but it
was not super useful, since it could not tell what was the unexpected
value that triggered the assertion. The stack trace was this:

  [583246.338097] assertion failed: !cache->nr_nodes, in fs/btrfs/backref.c:3158
  [583246.339588] ------------[ cut here ]------------
  [583246.340573] kernel BUG at fs/btrfs/backref.c:3158!
  [583246.342075] Oops: invalid opcode: 0000 [#1] SMP PTI
  [583246.343294] CPU: 5 UID: 0 PID: 677957 Comm: btrfs Not tainted 7.1.0-rc4-btrfs-next-234+ #1 PREEMPT(full)
  [583246.345715] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.16.2-0-gea1b7a073390-prebuilt.qemu.org 04/01/2014
  [583246.348694] RIP: 0010:btrfs_backref_release_cache.cold+0x61/0x84 [btrfs]
  [583246.350759] Code: 90 d5 7c (...)
  [583246.354923] RSP: 0018:ffffd4fc88c93ad8 EFLAGS: 00010246
  [583246.355982] RAX: 000000000000003e RBX: ffff8dec90d97020 RCX: 0000000000000000
  [583246.357459] RDX: 0000000000000000 RSI: 0000000000000001 RDI: 00000000ffffffff
  [583246.359517] RBP: ffff8dec8eeb78c0 R08: 0000000000000000 R09: 3fffffffffefffff
  [583246.361180] R10: ffffd4fc88c93970 R11: 0000000000000003 R12: ffff8decd21f3470
  [583246.363184] R13: 00000000fffffffe R14: ffff8decd21f3000 R15: ffff8decd21f3000
  [583246.364666] FS:  00007f9a51751400(0000) GS:ffff8df3f4255000(0000) knlGS:0000000000000000
  [583246.366287] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
  [583246.367443] CR2: 00007f9a518ed8f5 CR3: 00000004467c8002 CR4: 0000000000370ef0
  [583246.368969] Call Trace:
  [583246.369541]  <TASK>
  [583246.370040]  relocate_block_group+0xf2/0x520 [btrfs]
  [583246.371243]  btrfs_relocate_block_group+0x9a9/0x22e0 [btrfs]
  [583246.372443]  ? preempt_count_add+0x47/0xa0
  [583247.532978]  ? btrfs_tree_read_lock_nested+0x19/0x90 [btrfs]
  [583247.534520]  ? mutex_lock+0x1a/0x40
  [583247.602233]  ? btrfs_scrub_pause+0x2e/0x120 [btrfs]
  [583247.603543]  btrfs_relocate_chunk+0x3b/0x1a0 [btrfs]
  [583247.604893]  btrfs_balance+0x9d5/0x1920 [btrfs]
  [583247.606189]  ? preempt_count_add+0x69/0xa0
  [583247.607030]  btrfs_ioctl+0x260c/0x2a20 [btrfs]
  [583247.608015]  ? __memcg_slab_free_hook+0x156/0x1a0
  [583247.636971]  __x64_sys_ioctl+0x92/0xe0
  [583247.679247]  do_syscall_64+0x60/0xf20
  [583247.753297]  ? clear_bhb_loop+0x60/0xb0
  [583247.756321]  entry_SYSCALL_64_after_hwframe+0x76/0x7e
  [583247.787018] RIP: 0033:0x7f9a5186a8db
  [583247.787787] Code: 00 48 89 (...)
  [583247.791410] RSP: 002b:00007fff2ffa6ac0 EFLAGS: 00000246 ORIG_RAX: 0000000000000010
  [583247.792897] RAX: ffffffffffffffda RBX: 0000000000000003 RCX: 00007f9a5186a8db
  [583247.794319] RDX: 00007fff2ffa6bb0 RSI: 00000000c4009420 RDI: 0000000000000003
  [583247.795714] RBP: 0000000000000000 R08: 0000000000000000 R09: 0000000000000000
  [583247.797149] R10: 0000000000000000 R11: 0000000000000246 R12: 00007fff2ffa903f
  [583247.798685] R13: 00007fff2ffa6bb0 R14: 0000000000000002 R15: 0000000000000002
  [583247.800136]  </TASK>

So update all simple assertions in backref.c to print out the values when
they aren't testing simple boolean conditions.

Reviewed-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>