btrfs-progs: Dont' stop scanning of devices at first failed device

btrfs-progs: Dont' stop scanning of devices at first failed device

When 'btrfs device scan' command is invoked, it scans all devices,
check them for btrfs superblock and add devices with btrfs to a list.

Next, each device from the list is passed to kernel where it is handled
in the btrfs_scan_one_device() function. This function can, for example,
return -EBUSY when device contains superblock matched to existing and
mounted filesystem (if this device was pulled out from RAID and
connected again after some time).

btrfs tool stops device scan if any device has been failed to add, so
other existing devices with (possibly) valid FS will never be reached.

Fix this by remove stopping at any failure in the btrfs_register_all_devices(),
just return error count. btrfs_scan_one_device() reports any kind of
error already.

Signed-off-by: Yauhen Kharuzhy <yauhen.kharuzhy@zavadatar.com>
[ initialize err to 0 ]
Signed-off-by: David Sterba <dsterba@suse.com>