Use f_lock to protect f_flags

Traditionally, changes to struct file->f_flags have been done under BKL
protection, or with no protection at all.  This patch causes all f_flags
changes after file open/creation time to be done under protection of
f_lock.  This allows the removal of some BKL usage and fixes a number of
longstanding (if microscopic) races.

Reviewed-by: Christoph Hellwig <hch@lst.de>
Cc: Al Viro <viro@ZenIV.linux.org.uk>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
diff --git a/sound/oss/audio.c b/sound/oss/audio.c
index 89bd27a..b69c05b 100644
--- a/sound/oss/audio.c
+++ b/sound/oss/audio.c
@@ -433,7 +433,9 @@
 			return dma_ioctl(dev, cmd, arg);
 		
 		case SNDCTL_DSP_NONBLOCK:
+			spin_lock(&file->f_lock);
 			file->f_flags |= O_NONBLOCK;
+			spin_unlock(&file->f_lock);
 			return 0;
 
 		case SNDCTL_DSP_GETCAPS: