xfs: configurable behavior on errors at unmount time

xfs: configurable behavior on errors at unmount time

XFS used to retry forever on non-critical errors, and unmount could
hang in such case. Commit e6b3bb78962e ("xfs: add "fail at unmount"
error handling configuration") introduced an error configuration
option in sysfs(fail_at_unmount) and made this behavior
configurable.

Now test this "fail_at_unmount" behavior to make sure XFS doesn't
retry forever on error at unmount time, if configured so. Also
introduced new helpers to require/set/get sysfs attributes.

Signed-off-by: Zorro Lang <zlang@redhat.com>
Reviewed-by: Eryu Guan <eguan@redhat.com>
Signed-off-by: Eryu Guan <eguan@redhat.com>