xfs: use vmalloc instead of vm_map_area for buffer backing memory

The fallback buffer allocation path currently open codes a suboptimal
version of vmalloc to allocate pages that are then mapped into
vmalloc space.  Switch to using vmalloc instead, which uses all the
optimizations in the common vmalloc code, and removes the need to
track the backing pages in the xfs_buf structure.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Carlos Maiolino <cem@kernel.org>