btrfs-progs: send: fix handling of -c option

btrfs-progs: send: fix handling of -c option

When two or more -c options are specified, cannot find a suitable
parent. So, output stream is bigger than correct one.

[before]
At subvol Snap1
At subvol Snap2
At subvol ../SnapY
-rw------- 1 root root 3153 Oct 19 10:37 /tmp/data1

[after]
At subvol Snap1
At subvol Snap2
At subvol ../SnapY
-rw------- 1 root root 1492 Oct 19 10:39 /tmp/data1

Signed-off-by: Tsutomu Itoh <t-itoh@jp.fujitsu.com>
[ constify subvol argument, renamed single letter variables ]
Signed-off-by: David Sterba <dsterba@suse.com>