binfmt: clear the ignored flush failure before the registration loop

The "flush out all rules" step writes to /proc/sys/fs/binfmt_misc/status and
explicitly ignores failure, but it assigns the result to the same r that
RET_GATHER() consumes below. RET_GATHER() keeps the first error it is given,
so a failed flush is still returned from run() and systemd-binfmt exits 1 even
when every rule in binfmt.d was registered successfully.

Reset r after the ignored flush so it cannot be gathered.

Tested on WSL2 (kernel 6.18.40.1-microsoft-standard-WSL2), systemd 261, where
/proc/sys/fs/binfmt_misc/status is a read-only tmpfs. Before this, the binary
registered /etc/binfmt.d/nixos.conf successfully and then exited 1; after, it
exits 0. /proc/sys/fs/binfmt_misc/WSLInterop stays enabled in both cases.

Fixes #44035.

(cherry picked from commit 8fc6b2141b113f63427a766aa800539437138d6c)

src/binfmt/binfmt.c

@@ -219,6 +219,8 @@ static int run(int argc, char *argv[]) {
                 else
                         log_debug("Flushed all binfmt_misc rules.");
 
+                r = 0;
+
                 STRV_FOREACH(f, files)
                         RET_GATHER(r, apply_file(*f, true));
         }