staging: lustre: libcfs: Prevent harmless read underflow

Because this is a post-op instead of a pre-op, then it means we check
if knl_buffer[-1] is a space.  It doesn't really hurt anything, but
it causes a static checker warning so let's fix it.

Fixes: d7e09d0397e8 ("staging: add Lustre file system client support")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/lustre/lnet/libcfs/tracefile.c

@@ -785,7 +785,7 @@ int cfs_trace_copyin_string(char *knl_buffer, int knl_buffer_nob,
 		return -EFAULT;
 
 	nob = strnlen(knl_buffer, usr_buffer_nob);
-	while (nob-- >= 0)		      /* strip trailing whitespace */
+	while (--nob >= 0)		      /* strip trailing whitespace */
 		if (!isspace(knl_buffer[nob]))
 			break;