xfs: fix xfs to work with Virtually Indexed architectures

xfs_buf.c includes what is essentially a hand rolled version of
blk_rq_map_kern().  In order to work properly with the vmalloc buffers
that xfs uses, this hand rolled routine must also implement the flushing
API for vmap/vmalloc areas.

[style updates from hch@lst.de]
Acked-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: James Bottomley <James.Bottomley@suse.de>