nspawn: when readjusting UID/GID ownership of OS trees, skip read-only subtrees

This should allow tools like rkt to pre-mount read-only subtrees in the OS
tree, without breaking the patching code.

Note that the code will still fail, if the top-level directory is already
read-only.

src/basic/fd-util.c

@@ -25,11 +25,13 @@
 #include <unistd.h>
 
 #include "fd-util.h"
+#include "fs-util.h"
 #include "macro.h"
 #include "missing.h"
 #include "parse-util.h"
 #include "path-util.h"
 #include "socket-util.h"
+#include "stdio-util.h"
 #include "util.h"
 
 int close_nointr(int fd) {
@@ -356,3 +358,11 @@ bool fdname_is_valid(const char *s) {
 
         return p - s < 256;
 }
+
+int fd_get_path(int fd, char **ret) {
+        char procfs_path[strlen("/proc/self/fd/") + DECIMAL_STR_MAX(int)];
+
+        xsprintf(procfs_path, "/proc/self/fd/%i", fd);
+
+        return readlink_malloc(procfs_path, ret);
+}