btrfs-progs: completion: include files in "du" completion

Currently "btrfs filesystem du" auto-completes for directories only,
but it can also be used against files to determine shared vs exclusive
extents.

Signed-off-by: David Disseldorp <ddiss@suse.de>
Signed-off-by: David Sterba <dsterba@suse.com>

Author: ddiss

btrfs-completion

@@ -103,7 +103,7 @@ _btrfs()
 		case $cmd in
 			filesystem)
 				case $prev in
-					defragment)
+					du|defragment)
 						_filedir
 						return 0
 						;;