lightnvm: return the get_bb_tbl return value

During get_bb_tbl, a callback is used to allow an user-specific scan
function to be called. The callback may return an error, and in that
case, the return value is overridden. However, the callback error is
needed when the fault is a user error and not a kernel error. For
example, when a user tries to initialize the same device twice. The
get_bb_tbl callback should be able to communicate this.

Signed-off-by: Matias Bjørling <m@bjorling.me>
Signed-off-by: Jens Axboe <axboe@fb.com>