xfs: reuse _xfs_buf_read for re-reading the superblock

Instead of poking deeply into buffer cache internals when re-reading the
superblock during log recovery just generalize _xfs_buf_read and use it
there.  Note that we don't have to explicitly set up the ops as they
must be set from the initial read.

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>