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>

sound/oss/swarm_cs4297a.c

@@ -2200,7 +2200,9 @@ static int cs4297a_ioctl(struct inode *inode, struct file *file,
 				    sizeof(abinfo)) ? -EFAULT : 0;
 
 	case SNDCTL_DSP_NONBLOCK:
+		spin_lock(&file->f_lock);
 		file->f_flags |= O_NONBLOCK;
+		spin_unlock(&file->f_lock);
 		return 0;
 
 	case SNDCTL_DSP_GETODELAY: