ext4: clear buffer_uninit flag when submitting IO

Currently noone cleared buffer_uninit flag. This results in writeback
needlessly marking io_end as needing extent conversion scanning extent
tree for extents to convert. So clear the buffer_uninit flag once the
buffer is submitted for IO and the flag is transformed into
EXT4_IO_END_UNWRITTEN flag.

Signed-off-by: Jan Kara <jack@suse.cz>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
Reviewed-by: Zheng Liu <wenqing.lz@taobao.com>