wrappers for ->i_mutex access

parallel to mutex_{lock,unlock,trylock,is_locked,lock_nested},
inode_foo(inode) being mutex_foo(&inode->i_mutex).

Please, use those for access to ->i_mutex; over the coming cycle
->i_mutex will become rwsem, with ->lookup() done with it held
only shared.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

fs/fat/dir.c

@@ -769,7 +769,7 @@ static int fat_ioctl_readdir(struct inode *inode, struct file *file,
 
 	buf.dirent = dirent;
 	buf.result = 0;
-	mutex_lock(&inode->i_mutex);
+	inode_lock(inode);
 	buf.ctx.pos = file->f_pos;
 	ret = -ENOENT;
 	if (!IS_DEADDIR(inode)) {
@@ -777,7 +777,7 @@ static int fat_ioctl_readdir(struct inode *inode, struct file *file,
 				    short_only, both ? &buf : NULL);
 		file->f_pos = buf.ctx.pos;
 	}
-	mutex_unlock(&inode->i_mutex);
+	inode_unlock(inode);
 	if (ret >= 0)
 		ret = buf.result;
 	return ret;