dax_clear_blocks is currently performing a cond_resched() after every
PAGE_SIZE memset.  We need not check so frequently, for example md-raid
only calls cond_resched() at stripe granularity.  Also, in preparation
for introducing a dax_map_atomic() operation that temporarily pins a dax
mapping move the call to cond_resched() to the outer loop.

The worst case latency between calls to cond_resched() after this change
is 500us the average latency is 133us.  This is up from a 10us max and
4us average.
Signed-off-by: NDan Williams <dan.j.williams@intel.com>
Reviewed-by: NJan Kara <jack@suse.com>
Reviewed-by: NJeff Moyer <jmoyer@redhat.com>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>