jbd: Provide function to check whether transaction will issue data barrier

Provide a function which returns whether a transaction with given tid
will send a barrier to the filesystem device. The function will be used
by ext3 to detect whether fsync needs to send a separate barrier or not.

Signed-off-by: Jan Kara <jack@suse.cz>

diff --cc fs/jbd/commit.c
Simple merge

diff --cc fs/jbd/journal.c
Simple merge

diff --cc include/linux/jbd.h
Simple merge