The swap file must be set nocow before it is written to, otherwise it is
ignored and Btrfs refuses to activate it as swap.
Fixes: 25ce9740065e ("generic: test swapfile creation, activation, and deactivation")
Signed-off-by: Omar Sandoval <osandov@fb.com>
Reviewed-by: Eryu Guan <guaneryu@gmail.com>
Signed-off-by: Eryu Guan <guaneryu@gmail.com>