btrfs-progs: optimize not to scan repeated fsid mount points

btrfs-progs: optimize not to scan repeated fsid mount points

fsid can be mounted multiple times, with different subvolid.
And we don't have to scan a mount point if we already have
that in the scanned list.

And thus nicely avoids the following warning with multiple
subvol mounts on older kernel like 2.6.32 where
BTRFS_IOC_GET_FSLABEL ioctl does not exist.

./btrfs fi show -m
Label: none  uuid: 31845933-611e-422d-ae6f-386e57ad81aa
	Total devices 2 FS bytes used 172.00KiB
	devid    1 size 3.00GiB used 642.38MiB path /dev/sdd
	devid    2 size 3.00GiB used 622.38MiB path /dev/sde

warning, device 2 is missing
warning devid 2 not found already
warning, device 2 is missing
warning devid 2 not found already

Signed-off-by: Anand Jain <anand.jain@oracle.com>
Signed-off-by: David Sterba <dsterba@suse.com>