lightnvm: pblk: allow allocation of new lines during shutdown

When shutting down pblk the write buffer is flushed and if the
current line can't fit the data in the write buffer we need
to allocate a new line, so remove the check that prevents this.

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