ext3: Avoid starting a transaction in writepage when not necessary

We don't have to start a transaction in writepage() when all the blocks
are a properly allocated. Even in ordered mode either the data has been
written via write() and they are thus already added to transaction's list
or the data was written via mmap and then it's random in which transaction
they get written anyway.

This should help VM to pageout dirty memory without blocking on transaction
commits.

Signed-off-by: Jan Kara <jack@suse.cz>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>