xfs: fix unused variable warning in buffer completion on !DEBUG

The random buffer write failure errortag patch introduced a local
mount pointer variable for the test macro, but the macro is compiled
out on !DEBUG kernels. This results in an unused variable warning.

Access the mount structure through the buffer pointer and remove the
local mount pointer to address the warning.

Fixes: 7376d745 ("xfs: random buffer write failure errortag")
Signed-off-by: Brian Foster <bfoster@redhat.com>
Reviewed-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>