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/hostfs/hostfs_kern.c

@@ -378,9 +378,9 @@ static int hostfs_fsync(struct file *file, loff_t start, loff_t end,
 	if (ret)
 		return ret;
 
-	mutex_lock(&inode->i_mutex);
+	inode_lock(inode);
 	ret = fsync_file(HOSTFS_I(inode)->fd, datasync);
-	mutex_unlock(&inode->i_mutex);
+	inode_unlock(inode);
 
 	return ret;
 }