dm crypt: fix async inc_pending

The pending reference count must be incremented *before* the async work is
queued to another thread, not after.  Otherwise there's a race if the
work completes and decrements the reference count before it gets incremented.

Signed-off-by: Milan Broz <mbroz@redhat.com>
Signed-off-by: Alasdair G Kergon <agk@redhat.com>