fsx: introduce FSX_AVOID env var

fsx: introduce FSX_AVOID env var

Just like FSSTRESS_AVOID, FSX_AVOID can be used to add
options at the end of the default fsx runs in each test.
i.e. FSX_AVOID="-H -z -C" will disable punch hole, zero range,
and collapse range calls in all tests which run fsx.

This should handle Ted's concerns about buggy ext4 fallocate
code without needing to add tunables to the kernel to reject
these operations during xfstests runs.

Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>