lightnvm: pblk: remove spinlock when freeing line metadata

Lockdep complains about being in atomic context while freeing line
metadata - and rightly so as we take a spinlock and end up calling
vfree that might sleep(in pblk_mfree).

There is no need for holding the line manager free_lock while
freeing line metadata as the pipeline as stopped, so remove the lock.

Fixes: 588726d3 ("lightnvm: pblk: fail gracefully on irrec. error")
Signed-off-by: Hans Holmberg <hans.holmberg@cnexlabs.com>
Signed-off-by: Matias Bjørling <m@bjorling.me>
Signed-off-by: Jens Axboe <axboe@kernel.dk>