lightnvm: pblk: use vfree to free metadata on error path

As chunk metadata is allocated using vmalloc, we need to free it
using vfree.

Fixes: 090ee26f ("lightnvm: use internal allocation for chunk log page")
Signed-off-by: Hans Holmberg <hans.holmberg@cnexlabs.com>
Signed-off-by: Matias Bjørling <mb@lightnvm.io>
Signed-off-by: Jens Axboe <axboe@kernel.dk>