btrfs-progs: fix qgroup realloc inheritance

btrfs-progs: fix qgroup realloc inheritance

qgroup.c:82:23: warning: memcpy with byte count of 0
qgroup.c:83:23: warning: memcpy with byte count of 0

The inheritance wasn't copying qgroups[] because a confused sizeof()
gave 0 byte memcpy()s.  It's been like this for the year since it was
merged, so I guess this isn't a very important thing to do :).

Signed-off-by: Zach Brown <zab@redhat.com>
Reviewed-by: Arne Jansen <sensille@gmx.net>
Signed-off-by: David Sterba <dsterba@suse.cz>
Signed-off-by: Chris Mason <chris.mason@fusionio.com>