Linux 5.12 compat: META

Increase the Linux-Maximum version in the META file to 5.12.
All of the required compatibility patches have been merged.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>

META

@@ -6,5 +6,5 @@ Release:       1
 Release-Tags:  relext
 License:       CDDL
 Author:        OpenZFS
-Linux-Maximum: 5.11
+Linux-Maximum: 5.12
 Linux-Minimum: 3.10