generic/500: doesn't work for btrfs

generic/500: doesn't work for btrfs

Btrfs does COW, so when we unlink the file we need to update
metadata and write it to a new location, which we can't do because
the thinp is full.  This results in an EIO during a metadata write,
which makes us flip read only, thus making it impossible to fstrim
the fs.  Just make it so we skip this test for btrfs.

Signed-off-by: Josef Bacik <josef@toxicpanda.com>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Eryu Guan <guaneryu@gmail.com>