ext3_writepage() calls ext3_journal_stop(), which dereferences the affected
inode.

It does this _after_ writing the page out, which is illegal.  The IO can
complete, the page can be repeased from the inode and the inode can be freed
up.

It's a long-standing bug.  It has been reported happening on preemptible
kernels, where the timing window is larger.

Fix that up by teaching ext3_journal_stop to locate the superblock via the
journal structure, not via the inode.

This means that ext3_journal_stop() does not need the inode argument at all.

Also uninline the affected functions.  It saves 5.5 kbytes.

Also remove the setting of sb->s_dirt in ext3_journal_stop().  That was an
awkward way of telling sys_sync() that the filesystem needs a commit, and
with the ext3_sync_fs() that is no longer needed.