fix add_esp() in the gpt-auto-generator.c (#6251)

b9088048b1 seems to have broke it

fstab_is_mount_point() returns `true` (1) if the mount point exists and `false` (0) if it doesn't exist.

the change in b9088048 considered that if fstab_is_mount_point() returns 0
the mount point exists.

src/gpt-auto-generator/gpt-auto-generator.c

@@ -447,7 +447,7 @@ static int add_esp(DissectedPartition *p) {
         r = fstab_is_mount_point(esp);
         if (r < 0)
                 return log_error_errno(r, "Failed to parse fstab: %m");
-        if (r == 0) {
+        if (r > 0) {
                 log_debug("%s specified in fstab, ignoring.", esp);
                 return 0;
         }