Fix a race which was identified by Daniel McNeil <daniel@osdl.org>

If a buffer_head is under I/O due to JBD's ordered data writeout (which uses
ll_rw_block()) then either filemap_fdatawrite() or filemap_fdatawait() need
to wait on the buffer's existing I/O.

Presently neither will do so, because __block_write_full_page() will not
actually submit any I/O and will hence not mark the page as being under
writeback.

The best-performing fix would be to somehow mark the page as being under
writeback and defer waiting for the ll_rw_block-initiated I/O until
filemap_fdatawait()-time.  But this is hard, because in
__block_write_full_page() we do not have control of the buffer_head's end_io
handler.  Possibly we could make JBD call into end_buffer_async_write(), but
that gets nasty.

This patch makes __block_write_full_page() wait for any buffer_head I/O to
complete before inspecting the buffer_head state.  It only does this in the
case where __block_write_full_page() was called for a "data-integrity" write:
(wbc->sync_mode != WB_SYNC_NONE).

Probably it doesn't matter, because kjournald is currently submitting (or has
already submitted) all dirty buffers anyway.