jbd2: fix BH_JWrite setting in checkpointing code

BH_JWrite bit should be set when buffer is written to the journal. So
checkpointing shouldn't set this bit when writing out buffer. This didn't
cause any observable bug since BH_JWrite bit is used only for debugging
purposes but it's good to have this consistent.

Signed-off-by: Jan Kara <jack@suse.cz>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>