NFS: Replace __nfs_write_mapping with sync_inode()

Now that we have correct COMMIT semantics in writeback_single_inode, we can
reduce and simplify nfs_wb_all(). Also replace nfs_wb_nocommit() with a
call to filemap_write_and_wait(), which doesn't need to hold the
inode->i_mutex.

With that done, we can eliminate nfs_write_mapping() altogether.

Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>