Fix for #3160 MESH_BED_LEVELING broke correct G1 movement in Z

[epatel]

Now on branch RCBugFix.
See #3160
Not tested, but fairly certain this cover adding same linear fix for Z as for E.

[thinkyhead]

@epatel Does this patch need to be applied to dev also?

[epatel]

@thinkyhead dev? Maybe, I don't know what the current recommended "process" is. @Roxy-3DPrintBoard mentioned RCBugFix so I added it here.

[thinkyhead]

Well, at the moment the MarlinDev repository is in a holding pattern while we get RCBugFix patched up and ready for RC4. So whenever any change is made to RCBugFix I've been making an equivalent PR against the MarlinDev/dev branch, as an explicit reminder that the change still needs to be applied. At some later point I will need to compare the code between the two code-bases to find any changes that may have been missed. But I'd like to make sure that contributors get their due credit and Github reputation points for the commits they're contributing, so a PR by the original author is best.