web links to fixed spl issues are broken in zfs release notes

[ak100]

Web links for individual SPL issues are broken on zfs release log webpage starting with release 0.7.x . In older release notes the full url was used to refer spl issue. Apparently github assumes the issue linked by its ID is in the same (current) repository.

Example:

zfs-0.7.7
SPL
 Fix spl-kmod builds when using rpm >= 4.14 #691

The issue # 691 links to the issue #691 in zfs repository where it shall link to the issue #691 in spl repository:

current:
   https://github.com/zfsonlinux/zfs/pull/691
correct:
   https://github.com/zfsonlinux/spl/pull/691

Ways to fix: use url in form:
openzfs/spl#691 or #691 or spl #691

zfsonlinux/spl#691
[#691](zfsonlinux/spl#691)
[spl #691](zfsonlinux/spl#691)

[ak100]

I stand corrected :
Ways to fix - use full url in form:

openzfs/spl#691

Two other looked OK in preview but got rewritten to different URL when the issue was posted.

[behlendorf]

Manually fixed for the last few releases.