mm/truncate.c: fix build for CONFIG_BLOCK not enabled

Fix build error when CONFIG_BLOCK is not enabled by providing a stub
inode_dio_wait() function.

mm/truncate.c:612: error: implicit declaration of function 'inode_dio_wait'
Signed-off-by: Randy Dunlap <rdunlap@xenotime.net>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

include/linux/fs.h

@@ -2423,6 +2423,10 @@ static inline ssize_t blockdev_direct_IO(int rw, struct kiocb *iocb,
 				    offset, nr_segs, get_block, NULL, NULL,
 				    DIO_LOCKING | DIO_SKIP_HOLES);
 }
+#else
+static inline void inode_dio_wait(struct inode *inode)
+{
+}
 #endif
 
 extern const struct file_operations generic_ro_fops;