tree-wide: fix spelling of "fallback"

Similarly to "setup" vs. "set up", "fallback" is a noun, and "fall back"
is the verb. (This is pretty clear when we construct a sentence in the
present continous: "we are falling back" not "we are fallbacking").

src/basic/fd-util.c

@@ -976,6 +976,6 @@ int read_nr_open(void) {
                         return v;
         }
 
-        /* If we fail, fallback to the hard-coded kernel limit of 1024 * 1024. */
+        /* If we fail, fall back to the hard-coded kernel limit of 1024 * 1024. */
         return 1024 * 1024;
 }