ext4: silence warning in ext4_writepages()

The loop in mpage_map_and_submit_extent() is guaranteed to always run
at least once since the caller of mpage_map_and_submit_extent() makes
sure map->m_len > 0. So make that explicit using do-while instead of
pure while which also silences the compiler warning about
uninitialized 'err' variable.

Signed-off-by: Jan Kara <jack@suse.cz>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
Reviewed-by: Lukas Czerner <lczerner@redhat.com>

fs/ext4/inode.c

@@ -2163,7 +2163,7 @@ static int mpage_map_and_submit_extent(handle_t *handle,
 
 	mpd->io_submit.io_end->offset =
 				((loff_t)map->m_lblk) << inode->i_blkbits;
-	while (map->m_len) {
+	do {
 		err = mpage_map_one_extent(handle, mpd);
 		if (err < 0) {
 			struct super_block *sb = inode->i_sb;
@@ -2201,7 +2201,7 @@ static int mpage_map_and_submit_extent(handle_t *handle,
 		err = mpage_map_and_submit_buffers(mpd);
 		if (err < 0)
 			return err;
-	}
+	} while (map->m_len);
 
 	/* Update on-disk size after IO is submitted */
 	disksize = ((loff_t)mpd->first_page) << PAGE_CACHE_SHIFT;