xfs: use bios directly to read and write the log recovery buffers
The xfs_buf structure is basically used as a glorified container for
a memory allocation in the log recovery code. Replace it with a
call to kmem_alloc_large and a simple abstraction to read into or
write from it synchronously using chained bios.
Signed-off-by: Christoph Hellwig <hch@lst.de> Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com> Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>