ocfs2: fix ocfs2_sync_file() if filesystem is readonly

If filesystem is readonly, there is no need to flush drive's caches or
force any uncommitted transactions.

[akpm@linux-foundation.org: return -EROFS, not 0]
Signed-off-by: Younger Liu <younger.liucn@gmail.com>
Cc: Joel Becker <jlbec@evilplan.org>
Cc: Mark Fasheh <mfasheh@suse.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>