dm crypt: move dec_pending on error into write_io_submit

Make kcryptd_crypt_write_io_submit() responsible for decrementing
the pending count after an error.

Also fixes a bug in the async path that forgot to decrement it.

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