fs/dax.c: fix inefficiency in dax_writeback_mapping_range()

commit 1eb643d0 upstream.

dax_writeback_mapping_range() fails to update iteration index when
searching radix tree for entries needing cache flushing.  Thus each
pagevec worth of entries is searched starting from the start which is
inefficient and prone to livelocks.  Update index properly.

Link: http://lkml.kernel.org/r/20170619124531.21491-1-jack@suse.cz
Fixes: 9973c98e ("dax: add support for fsync/sync")
Signed-off-by: Jan Kara <jack@suse.cz>
Reviewed-by: Ross Zwisler <ross.zwisler@linux.intel.com>
Cc: Dan Williams <dan.j.williams@intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

fs/dax.c

@@ -875,6 +875,7 @@ int dax_writeback_mapping_range(struct address_space *mapping,
 			if (ret < 0)
 				return ret;
 		}
+		start_index = indices[pvec.nr - 1] + 1;
 	}
 	return 0;
 }