[LogFS] Only write journal if dirty

This prevents unnecessary journal writes.  More importantly it prevents
an oops due to a journal write on failed mount.

diff --cc fs/logfs/gc.c
Simple merge

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

diff --cc fs/logfs/logfs.h
Simple merge

diff --cc fs/logfs/readwrite.c
Simple merge

diff --cc fs/logfs/segment.c
Simple merge

diff --cc fs/logfs/super.c
Simple merge