drivers/syslog: fix syslog_write() returning -EIO on every write

syslog_write_foreach() compares an unsigned count against a signed
accumulator:

  size_t  nwritten     = 0;
  ssize_t nwritten_max = -EIO;
  ...
  if (nwritten > nwritten_max)
    {
      nwritten_max = nwritten;
    }
  return nwritten_max;

The usual arithmetic conversions promote nwritten_max to size_t, so -EIO
becomes 4294967291 on a 32-bit target, and the comparison is never true.
nwritten_max keeps its initial value and the function returns -EIO no
matter how many bytes actually went out.  Observed under gdb on a running
target: nwritten == 64, nwritten_max == -5, (nwritten > nwritten_max) == 0.

Most callers discard the result -- syslog() itself returns void -- so this
is normally invisible.  It becomes fatal when /dev/console is backed by
syslog_console_write(), because then stdio acts on it.
lib_fflush_unlocked() sees a negative return, sets __FS_FLAG_ERROR and
returns early, before resetting fs_bufpos.  The bytes have already been
emitted, but the buffer is never cleared, so every subsequent stdio call
re-flushes the same CONFIG_STDIO_BUFFER_SIZE bytes.  The console fills
with one repeated fragment and the system makes no further progress.

Reaching that state needs CONFIG_CONSOLE_SYSLOG=y together with no driver
claiming /dev/console ahead of syslog_console_init().  Three in-tree
defconfigs qualify: x86/qemu-i486:ostest, renesas/skp16c26:ostest and
x86_64/qemu-intel64:earlyfb.  The other 56 CONSOLE_SYSLOG configurations
have a serial console that registers /dev/console first, which is why this
has gone unnoticed.

Introduced by 1685e8ff7b ("syslog: avoid an infinite loop if one channel
fails"), which changed nwritten_max from size_t to ssize_t = -EIO so that
an all-channels-failed case could be reported.  Give nwritten the same type
so the comparison is signed, which preserves that intent: nwritten_max
stays -EIO only when no channel wrote anything.  nwritten is never negative,
so the remaining comparisons against buflen are unaffected.

Signed-off-by: Marco Casaroli <marco.casaroli@gmail.com>

Assisted-by: Claude Opus 5 (1M context) <noreply@anthropic.com>

drivers/syslog/syslog_write.c

@@ -106,7 +106,7 @@ ssize_t syslog_write_foreach(FAR const char *buffer,
 {
   syslog_write_t write;
   syslog_putc_t  putc;
-  size_t nwritten = 0;
+  ssize_t nwritten = 0;
   ssize_t nwritten_max = -EIO;
   ssize_t ret;
   int i;