linux-rt_5_9: export symbols needed by zfs

Upstream issue: https://github.com/openzfs/zfs/issues/11097#issuecomment-740682245

pkgs/os-specific/linux/kernel/export-rt-sched-migrate.patch

@@ -0,0 +1,11 @@
+Export linux-rt (PREEMPT_RT) specific symbols needed by ZFS.
+(Regular kernel provides them static inline in linux/preempt.h.)
+
+--- a/kernel/sched/core.c
++++ b/kernel/sched/core.c
+@@ -1812 +1812 @@ void migrate_disable(void)
+-EXPORT_SYMBOL_GPL(migrate_disable);
++EXPORT_SYMBOL(migrate_disable);
+@@ -1843 +1843 @@ void migrate_enable(void)
+-EXPORT_SYMBOL_GPL(migrate_enable);
++EXPORT_SYMBOL(migrate_enable);

pkgs/os-specific/linux/kernel/patches.nix

@@ -87,6 +87,11 @@
     };
   };
 
+  export-rt-sched-migrate = {
+    name = "export-rt-sched-migrate";
+    patch = ./export-rt-sched-migrate.patch;
+  };
+
   # patches from https://lkml.org/lkml/2019/7/15/1748
   mac_nvme_t2 = rec {
     name = "mac_nvme_t2";

pkgs/top-level/all-packages.nix

@@ -18363,6 +18363,7 @@ in
       kernelPatches.bridge_stp_helper
       kernelPatches.request_key_helper
       kernelPatches.export_kernel_fpu_functions."5.3"
+      kernelPatches.export-rt-sched-migrate
     ];
   };