xfs: allow scrubbers to pause background reclaim

The forthcoming summary counter patch races with regular filesystem
activity to compute rough expected values for the counters.  This design
was chosen to avoid having to freeze the entire filesystem to check the
counters, but while that's running we'd prefer to minimize background
reclamation activity to reduce the perturbations to the incore free
block count.  Therefore, provide a way for scrubbers to disable
background posteof and cowblock reclamation.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>