mdadm: Fix hardcoded directory

In [upstream change][1], handling of creating `/run/mdadm/creating-%s`
file was changed to fail if it was unable to create the file. This
revealed that the path itself incorrectly hardcodes `/run/mdadm` instead
of using `MAP_DIR` or similar. Since nixpkgs sets `RUN_DIR=/run/.mdadm`
at compile time, and `MAP_DIR=$(RUN_DIR)` in upstream, the `/run/mdadm`
is never created. This remedies by fixing the hard-coded directory.

[1]: https://git.kernel.org/pub/scm/utils/mdadm/mdadm.git/commit/?id=9f376da6439b07dc93ae084ab576e133b9d8d839

pkgs/os-specific/linux/mdadm/default.nix

@@ -9,7 +9,10 @@ stdenv.mkDerivation rec {
     sha256 = "sha256-QWcnrh8QgOpuMJDOo23QdoJvw2kVHjarc2VXupIZb58=";
   };
 
-  patches = [ ./no-self-references.patch ];
+  patches = [
+    ./no-self-references.patch
+    ./fix-hardcoded-mapdir.patch
+  ];
 
   makeFlags = [
     "NIXOS=1" "INSTALL=install" "BINDIR=$(out)/sbin"

pkgs/os-specific/linux/mdadm/fix-hardcoded-mapdir.patch

@@ -0,0 +1,13 @@
+diff --git a/udev.c b/udev.c
+index bc4722b0..aa2a1a24 100644
+--- a/udev.c
++++ b/udev.c
+@@ -167,7 +167,7 @@ enum udev_status udev_block(char *devnm)
+ 	int fd;
+ 	char *path = xcalloc(1, BUFSIZ);
+ 
+-	snprintf(path, BUFSIZ, "/run/mdadm/creating-%s", devnm);
++	snprintf(path, BUFSIZ, "%s/creating-%s", MAP_DIR, devnm);
+ 
+ 	fd = open(path, O_CREAT | O_RDWR, 0600);
+ 	if (!is_fd_valid(fd)) {