main: split out reading of /proc/sys/fs/nr_open into its own function

This doesn't really reduce the code size over all, but it does make main.c
shorter and more readable, and that's always a good thing.

src/core/main.c

@@ -1136,10 +1136,7 @@ static int prepare_reexecute(Manager *m, FILE **_f, FDSet **_fds, bool switching
 }
 
 static int bump_rlimit_nofile(struct rlimit *saved_rlimit) {
-        struct rlimit nl;
-        int r;
-        int min_max;
-        _cleanup_free_ char *nr_open = NULL;
+        int r, nr;
 
         assert(saved_rlimit);
 
@@ -1160,17 +1157,9 @@ static int bump_rlimit_nofile(struct rlimit *saved_rlimit) {
                 arg_default_rlimit[RLIMIT_NOFILE] = rl;
         }
 
-        /* Get current RLIMIT_NOFILE maximum compiled into the kernel. */
-        r = read_one_line_file("/proc/sys/fs/nr_open", &nr_open);
-        if (r >= 0)
-                r = safe_atoi(nr_open, &min_max);
-        /* If we fail, fallback to the hard-coded kernel limit of 1024 * 1024. */
-        if (r < 0)
-                min_max = 1024 * 1024;
-
-        /* Bump up the resource limit for ourselves substantially */
-        nl.rlim_cur = nl.rlim_max = min_max;
-        r = setrlimit_closest(RLIMIT_NOFILE, &nl);
+        /* Bump up the resource limit for ourselves substantially, all the way to the maximum the kernel allows */
+        nr = read_nr_open();
+        r = setrlimit_closest(RLIMIT_NOFILE, &RLIMIT_MAKE_CONST(nr));
         if (r < 0)
                 return log_warning_errno(r, "Setting RLIMIT_NOFILE failed, ignoring: %m");