libceph: clean up ceph_osdc_start_request prototype

This function always returns 0, and ignores the nofail boolean. Drop the
nofail argument, make the function void return and fix up the callers.

Signed-off-by: Jeff Layton <jlayton@kernel.org>
Reviewed-by: Ilya Dryomov <idryomov@gmail.com>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>