ext4: Restore wbc->range_start in ext4_da_writepages()

To solve a lock inversion problem, we implement part of the
range_cyclic algorithm in ext4_da_writepages().  (See commit 2acf2c26
for more details.)

As part of that change wbc->range_start was modified by ext4's
writepages function, which causes its callers to get confused since
they aren't expecting the filesystem to modify it.  The simplest fix
is to save and restore wbc->range_start in ext4_da_writepages.
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>

fs/ext4/inode.c

@@ -2749,6 +2749,7 @@ static int ext4_da_writepages(struct address_space *mapping,
 	long pages_skipped;
 	int range_cyclic, cycled = 1, io_done = 0;
 	int needed_blocks, ret = 0, nr_to_writebump = 0;
+	loff_t range_start = wbc->range_start;
 	struct ext4_sb_info *sbi = EXT4_SB(mapping->host->i_sb);
 
 	trace_ext4_da_writepages(inode, wbc);
@@ -2917,6 +2918,7 @@ out_writepages:
 	if (!no_nrwrite_index_update)
 		wbc->no_nrwrite_index_update = 0;
 	wbc->nr_to_write -= nr_to_writebump;
+	wbc->range_start = range_start;
 	trace_ext4_da_writepages_result(inode, wbc, ret, pages_written);
 	return ret;
 }