lightnvm: pblk: fix resource leak of invalid_bitmap

Currently the error exit path when the emeta could not be
interpreted is via fail_free_ws and this fails to free
invalid_bitmap.  Fix this by adding another exit label and
exiting via this to kfree invalid_bitmap.

Detected by CoverityScan, CID#1469659 ("Resource leak")

Fixes: 48b8d208 ("lightnvm: pblk: garbage collect lines with failed writes")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>