Update kernel documentation links

This patch originally intended to just replace github.com links with kernel.org ones, but it turned out the existing addresses were actually outdated due to upstream commit 39443104c7d3f2b05a4a330fbcef6da68f80d60b ("docs: blockdev: convert to ReST") and commit e7751617dd0599ceadf4221cb08e04307b00aa1f ("docs: blockdev: add it to the admin-guide"). Replace the github.com zram.txt link with a current git.kernel.org link. Replace the link to the compression algorithm documentation with a www.kernel.org/doc link.