libceph: unregister request in __map_request failed and nofail == false

commit 73d9f7eef3d98c3920e144797cc1894c6b005a1e upstream.

For nofail == false request, if __map_request failed, the caller does
cleanup work, like releasing the relative pages.  It doesn't make any sense
to retry this request.

Signed-off-by: Jianpeng Ma <majianpeng@gmail.com>
Reviewed-by: Sage Weil <sage@inktank.com>
[bwh: Backported to 3.2: adjust indentation]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>