AP_Logger: try harder to use close to user buffer size

this stops us drastically reducing buffer size when low on memory

libraries/AP_Logger/AP_Logger_File.cpp

@@ -83,10 +83,14 @@ void AP_Logger_File::Init()
     }
     bufsize *= 1024;
 
+    const uint32_t desired_bufsize = bufsize;
+
     // If we can't allocate the full size, try to reduce it until we can allocate it
     while (!_writebuf.set_size(bufsize) && bufsize >= _writebuf_chunk) {
-        hal.console->printf("AP_Logger_File: Couldn't set buffer size to=%u\n", (unsigned)bufsize);
-        bufsize >>= 1;
+        bufsize *= 0.9;
+    }
+    if (bufsize >= _writebuf_chunk && bufsize != desired_bufsize) {
+        hal.console->printf("AP_Logger: reduced buffer %u/%u\n", (unsigned)bufsize, (unsigned)desired_bufsize);
     }
 
     if (!_writebuf.get_size()) {