dm thin metadata: decrement counter after removing mapped block

commit af63bcb817cf708f53bcae6edc2e3fb7dd7d8051 upstream.

Correct the number of mapped sectors shown on a thin device's
status line by decrementing td->mapped_blocks in __remove() each time
a block is removed.

Signed-off-by: Joe Thornber <ejt@redhat.com>
Acked-by: Mike Snitzer <snitzer@redhat.com>
Signed-off-by: Alasdair G Kergon <agk@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>