lightnvm: pblk: protect line bitmap while submitting meta io

It seems pblk_dealloc_page would race against pblk_alloc_pages for
line bitmap for sector allocation.The chances are very low but might
as well protect the bitmap properly.

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