[XFS] avoid race in sync_inodes() that can fail to write out all dirty data

In xfs_fs_sync_super() treat a sync the same as a filesystem freeze. This
is needed to force the log to disk for inodes which are not marked dirty
in the Linux inode (the inodes are marked dirty on completion of the log
I/O) and so sync_inodes() will not flush them.

In xfs_fs_write_inode() a synchronous flush will not get an EAGAIN from
xfs_inode_flush() and if an asynchronous flush returns EAGAIN we should
pass it on to the caller. If we get an error while flushing the inode then
re-dirty it so we can try again later.

SGI-PV: 971670
SGI-Modid: xfs-linux-melb:xfs-kern:29860a

Signed-off-by: Lachlan McIlroy <lachlan@sgi.com>
Signed-off-by: David Chinner <dgc@sgi.com>
Signed-off-by: Tim Shimmin <tes@sgi.com>