xtensa/nputs: add a mutex to prevent concurrent tasks from writing

Considering SMP, multiples CPU would be able to call up_nputs
simultaneously. This function is most used by syslog and, in spite
of the syslog supporting buffered output, the implementation of
the up_nputs allows multiple threads to run concurrently, messing
with the log messages.

Please check apache#6618 for more
information about the locking mechanism.

arch/xtensa/src/common/xtensa_nputs.c

@@ -24,6 +24,7 @@
 
 #include <nuttx/config.h>
 #include <nuttx/arch.h>
+#include <nuttx/mutex.h>
 
 /****************************************************************************
  * Public Functions
@@ -39,8 +40,14 @@
 
 void up_nputs(const char *str, size_t len)
 {
+  static mutex_t lock = NXMUTEX_INITIALIZER;
+
+  nxmutex_lock(&lock);
+
   while (len-- > 0 && *str)
     {
       up_putc(*str++);
     }
+
+  nxmutex_unlock(&lock);
 }