Add expected failures for ZFS

- :f_flag fails on ZFS because UF_IMMUTABLE isn't supported.
- :v_flag fails on ZFS because the mode for foo is [always] updated
  unnecessarily.

get_filesystem(..) (supporting function that was added to the test
script) is based on equivalent logic in
usr.bin/extattr/tests/extattr_test.sh .

Always use first parameter passed to get_filesystem(..) instead of
discarding it and using `.` instead.

Don't check result of chflags in f_flag_cleanup()

This will prevent false positives from occurring if the test is run
on ZFS since ZFS doesn't support fflags throbbing like UFS.