generic implementaion and changes fs-writeback.c:sync_sb_inodes() to call
filesystem's sync_inodes if it is defined or generic implementaion otherwise.
This new operation allows filesystem to decide itself what to flush.

Reiser4 flushes dirty pages on basic of atoms, not of inodes.  sync_sb_inodes
used to call address space flushing method (writepages) for every dirty inode.
 For reiser4 it caused having to commit atoms unnecessarily often.  This
turned into substantial slowdown.  Having this method helped to fix that
problem.

akpm: this patch needs to be chnaged to remove the `sb' arg.
Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>
Cc: Edward Shishkin <edward.shishkin@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>