dm crypt: tidy write loop pending

Move kcryptd_crypt_write_convert_loop inside kcryptd_crypt_write_convert.
This change is needed for a later patch.

No functional change.

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