[XFS] Fix fsync() b0rkage.

xfs_fsync() fails to wait for data I/O completion before checking if the
inode is dirty or clean to decide whether to log the inode or not. This
misses inode size updates when the data flushed by the fsync() is
extending the file.

Hence, like fdatasync(), we need to wait for I/o completion first, then
check the inode for cleanliness. Doing so makes the behaviour of
xfs_fsync() identical for fsync and fdatasync and we *always* use
synchronous semantics if the inode is dirty. Therefore also kill the
differences and remove the unused flags from the xfs_fsync function and
callers.

SGI-PV: 981296
SGI-Modid: xfs-linux-melb:xfs-kern:31033a

Signed-off-by: David Chinner <dgc@sgi.com>
Signed-off-by: Christoph Hellwig <hch@infradead.org>
Signed-off-by: Lachlan McIlroy <lachlan@sgi.com>