btrfs: fix false EIO for missing device

When one of the device is missing, bbio_error() takes care of setting
the error status. And if its only IO that is pending in that stripe, it
fails to check the status of the other IO at %bbio_error before setting
the error %bi_status for the %orig_bio. Fix this by checking if
%bbio->error has exceeded the %bbio->max_errors.

Reproducer as below fdatasync error is seen intermittently.

 mount -o degraded /dev/sdc /btrfs
 dd status=none if=/dev/zero of=$(mktemp /btrfs/XXX) bs=4096 count=1 conv=fdatasync

 dd: fdatasync failed for ‘/btrfs/LSe’: Input/output error

 The reason for the intermittences of the problem is because
 the following conditions have to be met, which depends on timing:
 In btrfs_map_bio()
  - the RAID1 the missing device has to be at %dev_nr = 1
 In bbio_error()
  . before bbio_error() is called the bio of the not-missing
    device at %dev_nr = 0 must be completed so that the below
    condition is true
     if (atomic_dec_and_test(&bbio->stripes_pending)) {

Signed-off-by: Anand Jain <anand.jain@oracle.com>
Reviewed-by: Liu Bo <bo.li.liu@oracle.com>
Signed-off-by: David Sterba <dsterba@suse.com>