lightnvm: pblk: use constant for GC max inflight

Use a constant to set the maximum number of inflight GC requests
allowed.

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