Bump to 5.1.5

Fix fstrim bug. See:
- https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?h=linux-5.1.y&id=871e122d55e8d1cd7c0d5dec9bdba1fe45406196
- https://www.phoronix.com/scan.php?page=news_item&px=Linux-5.1-FSTRIM-Bug
- https://www.reddit.com/r/archlinux/comments/brbvo7/psa_fstrim_discarding_too_many_or_wrong_blocks_on/
- https://bugs.archlinux.org/task/62693