Linux 5.15 compat: META (openzfs#12824)

The final 5.15 kernel is available and has been tested. 

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: George Melikov <mail@gmelikov.ru>
Reviewed-by: Tony Hutter <hutter2@llnl.gov>

META

@@ -6,5 +6,5 @@ Release:       1
 Release-Tags:  relext
 License:       CDDL
 Author:        OpenZFS
-Linux-Maximum: 5.14
+Linux-Maximum: 5.15
 Linux-Minimum: 3.10