btrfs-progs: there is devid 0 when replace is running

btrfs-progs: there is devid 0 when replace is running

as of now, when we replace a disk, it is added to the
dev list with devid 0. And we fail to obtain details
of devid 0 because we don't query devid 0 at all.

reproducer:

 btrfs rep start /dev/sdb /dev/sdf /btrfs
 btrfs fi show
 Label: none  uuid: f8fb9819-16c8-47b7-b62f-0ff90f8c56cd
        Total devices 3 FS bytes used 1.94GiB
        devid    1 size 1.10GiB used 1.10GiB path /dev/sdb
        devid    2 size 1.10GiB used 1.08GiB path /dev/sdc
        devid    0 size 0.00 used 0.00 path

  this patch will make it proper by querying devid 0.

 btrfs repl start /dev/sdb /dev/sdf /btrfs
 btrfs fi show /btrfs
 Label: none  uuid: f8fb9819-16c8-47b7-b62f-0ff90f8c56cd
        Total devices 3 FS bytes used 1.94GiB
        devid    0 size 1.10GiB used 1.10GiB path /dev/sdf
        devid    1 size 1.10GiB used 1.10GiB path /dev/sdb
        devid    2 size 1.10GiB used 1.08GiB path /dev/sdc

 Its fine to query devid 0 when there is no replace
 activity as well, because we just skip the error ENODEV

 btrfs fi show /btrfs
 Label: none  uuid: f8fb9819-16c8-47b7-b62f-0ff90f8c56cd
        Total devices 2 FS bytes used 1.94GiB
        devid    1 size 1.10GiB used 1.10GiB path /dev/sdf
        devid    2 size 1.10GiB used 1.08GiB path /dev/sdc

Signed-off-by: Anand Jain <Anand.Jain@oracle.com>
Signed-off-by: David Sterba <dsterba@suse.cz>
Signed-off-by: Chris Mason <clm@fb.com>