xfs: reimplement fdatasync support

Add an in-memory only flag to say we logged timestamps only, and use it to
check if fdatasync can optimize away the log force.

Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Mark Tinguely <tinguely@sgi.com>
Signed-off-by: Ben Myers <bpm@sgi.com>

diff --cc fs/xfs/xfs_file.c
Simple merge

diff --cc fs/xfs/xfs_inode_item.c
Simple merge

diff --cc fs/xfs/xfs_inode_item.h
Simple merge

diff --cc fs/xfs/xfs_super.c
Simple merge