systemctl/core: ignore masked units in preset-all

With any masked unit that would that would be enabled by presets, we'd get:

test@rawhide $ sudo systemctl preset-all
Failed to execute operation: Unit file is masked.

test@rawhide $ sudo systemctl --root=/ preset-all
Operation failed: Cannot send after transport endpoint shutdown

Simply ignore those units:

test@rawhide $ sudo systemctl preset-all
Unit xxx.service is masked, ignoring.

1 files changed, 12 insertions, 4 deletions

diff --git a/src/systemctl/systemctl.c b/src/systemctl/systemctl.c
index 4942dbb6ad..0c64cbab80 100644
--- a/src/systemctl/systemctl.c
+++ b/src/systemctl/systemctl.c
@@ -1989,12 +1989,20 @@ static void dump_unit_file_changes(const UnitFileChange *changes, unsigned n_cha
 
         assert(changes || n_changes == 0);
 
-        for (i = 0; i < n_changes; i++) {
-                if (changes[i].type == UNIT_FILE_SYMLINK)
+        for (i = 0; i < n_changes; i++)
+                switch(changes[i].type) {
+                case UNIT_FILE_SYMLINK:
                         log_info("Created symlink %s, pointing to %s.", changes[i].path, changes[i].source);
-                else
+                        break;
+                case UNIT_FILE_UNLINK:
                         log_info("Removed %s.", changes[i].path);
-        }
+                        break;
+                case UNIT_FILE_IS_MASKED:
+                        log_info("Unit %s is masked, ignoring.", changes[i].path);
+                        break;
+                default:
+                        assert_not_reached("bad change type");
+                }
 }
 
 static int set_default(int argc, char *argv[], void *userdata) {