generic: add a regression test for sub-block fsmap queries

Zizhi Wo found some bugs in the GETFSMAP implementation if it is fed
sub-fsblock ranges.  Add a regression test for this.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Zorro Lang <zlang@redhat.com>
Signed-off-by: Zorro Lang <zlang@kernel.org>