lightnvm: pblk: refactor read path on GC

Simplify the part of the garbage collector where data is read from the
line being recycled and moved into an internal queue before being copied
to the memory buffer. This allows to get rid of a dedicated function,
which introduces an unnecessary dependency on the code.

Signed-off-by: Javier González <javier@cnexlabs.com>
Signed-off-by: Matias Bjørling <m@bjorling.me>
Signed-off-by: Jens Axboe <axboe@kernel.dk>