xfs: constify the buffer pointer arguments to error functions

Some of the xfs error message functions take a pointer to a buffer that
will be dumped to the system log.  The logging functions don't change
the contents, so constify all the parameters.  This enables the next
patch to ensure that we log bad metadata when we encounter it.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Carlos Maiolino <cmaiolino@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>