generic/507: support more filesystems

generic/507: support more filesystems

The commit message introducing the test says:
"We only check below attribute modification which most filesystem
 supports:
    - no atime updates (A)
    - secure deletion (s)
    - synchronous updates (S)
    - undeletable (u)
"
But in fact, very few filesystems support the (s) and (u) flags.
xfs and btrfs do not support them for example.

The test doesn't need to check those specific flags, so replace those
flags with immutable (i) and append-only (a), which most filesystems
really do support.

Signed-off-by: Amir Goldstein <amir73il@gmail.com>
Reviewed-by: Eryu Guan <guaneryu@gmail.com>
Signed-off-by: Eryu Guan <guaneryu@gmail.com>