btrfs: check for missing device in btrfs_trim_fs

A fstrim on a degraded raid1 can trigger the following null pointer
dereference:

  BTRFS info (device loop0): allowing degraded mounts
  BTRFS info (device loop0): disk space caching is enabled
  BTRFS info (device loop0): has skinny extents
  BTRFS warning (device loop0): devid 2 uuid 97ac16f7-e14d-4db1-95bc-3d489b424adb is missing
  BTRFS warning (device loop0): devid 2 uuid 97ac16f7-e14d-4db1-95bc-3d489b424adb is missing
  BTRFS info (device loop0): enabling ssd optimizations
  BUG: kernel NULL pointer dereference, address: 0000000000000620
  PGD 0 P4D 0
  Oops: 0000 [#1] SMP NOPTI
  CPU: 0 PID: 4574 Comm: fstrim Not tainted 5.13.0-rc7+ #31
  Hardware name: innotek GmbH VirtualBox/VirtualBox, BIOS VirtualBox 12/01/2006
  RIP: 0010:btrfs_trim_fs+0x199/0x4a0 [btrfs]
  RSP: 0018:ffff959541797d28 EFLAGS: 00010293
  RAX: 0000000000000000 RBX: ffff946f84eca508 RCX: a7a67937adff8608
  RDX: ffff946e8122d000 RSI: 0000000000000000 RDI: ffffffffc02fdbf0
  RBP: ffff946ea4615000 R08: 0000000000000001 R09: 0000000000000000
  R10: 0000000000000000 R11: ffff946e8122d960 R12: 0000000000000000
  R13: ffff959541797db8 R14: ffff946e8122d000 R15: ffff959541797db8
  FS:  00007f55917a5080(0000) GS:ffff946f9bc00000(0000) knlGS:0000000000000000
  CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
  CR2: 0000000000000620 CR3: 000000002d2c8001 CR4: 00000000000706f0
  Call Trace:
  btrfs_ioctl_fitrim+0x167/0x260 [btrfs]
  btrfs_ioctl+0x1c00/0x2fe0 [btrfs]
  ? selinux_file_ioctl+0x140/0x240
  ? syscall_trace_enter.constprop.0+0x188/0x240
  ? __x64_sys_ioctl+0x83/0xb0
  __x64_sys_ioctl+0x83/0xb0

Reproducer:

  $ mkfs.btrfs -fq -d raid1 -m raid1 /dev/loop0 /dev/loop1
  $ mount /dev/loop0 /btrfs
  $ umount /btrfs
  $ btrfs dev scan --forget
  $ mount -o degraded /dev/loop0 /btrfs

  $ fstrim /btrfs

The reason is we call btrfs_trim_free_extents() for the missing device,
which uses device->bdev (NULL for missing device) to find if the device
supports discard.

Fix is to check if the device is missing before calling
btrfs_trim_free_extents().

CC: stable@vger.kernel.org # 5.4+
Reviewed-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: Anand Jain <anand.jain@oracle.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>