xfs: stop using bp naming for log recovery buffers

Now that we don't use struct xfs_buf to hold log recovery buffer rename
the related functions and variables to just talk of a buffer instead of
using the bp name that we usually use for xfs_buf related functionality.
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>