xfs: optimize bio handling in the buffer writeback path

This patch implements two closely related changes:  First it embeds
a bio the ioend structure so that we don't have to allocate one
separately.  Second it uses the block layer bio chaining mechanism
to chain additional bios off this first one if needed instead of
manually accounting for multiple bio completions in the ioend
structure.  Together this removes a memory allocation per ioend and
greatly simplifies the ioend setup and I/O completion path.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Brian Foster <bfoster@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>