SYSLOG_DEFAULT: wrap up_putc/up_nputs calls with critical section

This would avoid the undesirable intertactions with the serial driver
described in #14662.

Although I'm not entirely happy with this fix because it assumes
the particular implementations of up_putc/up_nputc and its association
to the serial devices, I haven't come up with better ideas for now.

An alternative is to place some serializations inside the target
specific serial (and/or whatever provides up_putc api) implementaitons.
But it isn't too attractive to put potentially complex logic into the
low-level machinaries, especially when we have a lot of similar copies
of it.

Another alternative is to deprecate up_putc. (at least for the purpose
of syslog.) But it seems at least some of users are relying on what
the current implementation provides heavily.

This commit also removes g_lowputs_lock because the critical section
would serve the purpose of the lock as well.

drivers/serial/serial.c

@@ -268,6 +268,10 @@ static int uart_putxmitchar(FAR uart_dev_t *dev, int ch, bool oktoblock)
         {
           /* The following steps must be atomic with respect to serial
            * interrupt handling.
+           *
+           * This critical section is also used for the serialization
+           * with the up_putc-based syslog channels.
+           * See https://github.com/apache/nuttx/issues/14662
            */
 
           flags = enter_critical_section();

drivers/syslog/syslog_channel.c

@@ -33,7 +33,6 @@
 
 #include <nuttx/syslog/syslog.h>
 #include <nuttx/compiler.h>
-#include <nuttx/mutex.h>
 
 #ifdef CONFIG_RAMLOG_SYSLOG
 #  include <nuttx/syslog/ramlog.h>
@@ -72,10 +71,6 @@ static ssize_t syslog_default_write(FAR syslog_channel_t *channel,
  * Private Data
  ****************************************************************************/
 
-#if defined(CONFIG_SYSLOG_DEFAULT) && defined(CONFIG_ARCH_LOWPUTC)
-static mutex_t g_lowputs_lock = NXMUTEX_INITIALIZER;
-#endif
-
 #ifdef CONFIG_RAMLOG_SYSLOG
 static const struct syslog_channel_ops_s g_ramlog_channel_ops =
 {
@@ -234,23 +229,31 @@ g_syslog_channel[CONFIG_SYSLOG_MAX_CHANNELS] =
 #ifdef CONFIG_SYSLOG_DEFAULT
 static int syslog_default_putc(FAR syslog_channel_t *channel, int ch)
 {
-  UNUSED(channel);
-
 #  ifdef CONFIG_ARCH_LOWPUTC
+  /* See https://github.com/apache/nuttx/issues/14662
+   * about what this critical section is for.
+   */
+
+  irqstate_t flags = enter_critical_section();
   up_putc(ch);
+  leave_critical_section(flags);
 #  endif
+
+  UNUSED(channel);
   return ch;
 }
 
 static ssize_t syslog_default_write(FAR syslog_channel_t *channel,
                                     FAR const char *buffer, size_t buflen)
 {
 #  ifdef CONFIG_ARCH_LOWPUTC
-  nxmutex_lock(&g_lowputs_lock);
+  /* See https://github.com/apache/nuttx/issues/14662
+   * about what this critical section is for.
+   */
 
+  irqstate_t flags = enter_critical_section();
   up_nputs(buffer, buflen);
-
-  nxmutex_unlock(&g_lowputs_lock);
+  leave_critical_section(flags);
 #  endif
 
   UNUSED(channel);