xfs: refactor scrub context initialization

It's a little silly how the memset in scrub context initialization
forces us to declare stack variables to preserve context variables
across a retry.  Since the teardown functions already null out most of
the ephemeral state (buffer pointers, btree cursors, etc.), just skip
the memset and move the initialization as needed.

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