new helper: file_inode(file)

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
diff --git a/drivers/char/mem.c b/drivers/char/mem.c
index c6fa3bc..e23b4a2 100644
--- a/drivers/char/mem.c
+++ b/drivers/char/mem.c
@@ -708,7 +708,7 @@
 {
 	loff_t ret;
 
-	mutex_lock(&file->f_path.dentry->d_inode->i_mutex);
+	mutex_lock(&file_inode(file)->i_mutex);
 	switch (orig) {
 	case SEEK_CUR:
 		offset += file->f_pos;
@@ -725,7 +725,7 @@
 	default:
 		ret = -EINVAL;
 	}
-	mutex_unlock(&file->f_path.dentry->d_inode->i_mutex);
+	mutex_unlock(&file_inode(file)->i_mutex);
 	return ret;
 }