btrfs: proper error handling when invalid device is found in find_next_devid

In a corrupted tree, if search for next devid finds the device with
devid = -1, then report the error -EUCLEAN back to the parent function
to fail gracefully.

The tree checker will not catch this in case the devids are created
using the following script:

  umount /btrfs
  dev1=/dev/sdb
  dev2=/dev/sdc
  mkfs.btrfs -fq -dsingle -msingle $dev1
  mount $dev1 /btrfs

  _fail()
  {
	  echo $1
	  exit 1
  }

  while true; do
	  btrfs dev add -f $dev2 /btrfs || _fail "add failed"
	  btrfs dev del $dev1 /btrfs || _fail "del failed"
	  dev_tmp=$dev1
	  dev1=$dev2
	  dev2=$dev_tmp
  done

With output:

  BTRFS critical (device sdb): corrupt leaf: root=3 block=313739198464 slot=1 devid=1 invalid devid: has=507 expect=[0, 506]
  BTRFS error (device sdb): block=313739198464 write time tree block corruption detected
  BTRFS: error (device sdb) in btrfs_commit_transaction:2268: errno=-5 IO failure (Error while writing out transaction)
  BTRFS warning (device sdb): Skipping commit of aborted transaction.
  BTRFS: error (device sdb) in cleanup_transaction:1827: errno=-5 IO failure

Reviewed-by: Johannes Thumshirn <jthumshirn@suse.de>
Reviewed-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: Nikolay Borisov <nborisov@suse.com>
Signed-off-by: Anand Jain <anand.jain@oracle.com>
[ add script and messages ]
Signed-off-by: David Sterba <dsterba@suse.com>