fstab-generator: ignore root=/dev/nfs (#3591)

root=/dev/nfs is a legacy option for the kernel to handle root on NFS.

Documentation for this kernel command line option
can be found in the kernel source tree:

Documentation/filesystems/nfs/nfsroot.txt

src/fstab-generator/fstab-generator.c

@@ -502,6 +502,12 @@ static int add_sysroot_mount(void) {
                 return 0;
         }
 
+        if (streq(arg_root_what, "/dev/nfs")) {
+                /* This is handled by the kernel or the initrd */
+                log_debug("Skipping root directory handling, as /dev/nfs was requested.");
+                return 0;
+        }
+
         what = fstab_node_to_udev_node(arg_root_what);
         if (!what)
                 return log_oom();