fstests: add module reloading helpers

fstests: add module reloading helpers

Add some helper functions to require that we can reload a given
module, and add a helper to actually do that.  Refactor the existing
users to use the generics.

We need to hoist completely the behaviors of the old btrfs module
helper because we need to confirm before starting the test that we
actually can remove the module.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Eryu Guan <eguan@redhat.com>
Signed-off-by: Eryu Guan <eguan@redhat.com>