btrfs/173: make the test work when mounting with nodatacow

btrfs/173: make the test work when mounting with nodatacow

Currently btrfs/173 fails when passing "-o nodatacow" to MOUNT_OPTIONS
because it assumes that when creating a file it does not have the
nodatacow flag set, which is obviously not true if the fs is mounted with
"-o nodatacow". To allow the test to run successfully with nodatacow,
just make sure it clears the nodatacow flag from the file if the fs was
mounted with "-o nodatacow".

Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: David Disseldorp <ddiss@suse.de>
Signed-off-by: Zorro Lang <zlang@kernel.org>