lightnvm: pblk: gracefully handle GC vmalloc fail

Currently when we fail on rq data allocation in gc, it skips moving
active data and moves line straigt to its free state. Losing user
data in the process.

Move the data allocation to an earlier phase of GC, where we can still
fail gracefully by moving line back to the closed state.

Signed-off-by: Igor Konopko <igor.j.konopko@intel.com>
Reviewed-by: Javier González <javier@javigon.com>
Reviewed-by: Hans Holmberg <hans.holmberg@cnexlabs.com>
Signed-off-by: Matias Bjørling <mb@lightnvm.io>
Signed-off-by: Jens Axboe <axboe@kernel.dk>