xfs: online scrub needn't bother zeroing its temporary bufferattr-scrub-fixes_2019-06-26

The xattr scrubber functions use the temporary memory buffer either for
storing bitmaps or for testing if attribute value extraction works.  The
bitmap code always zeroes what it needs and the value extraction merely
sets the buffer contents (we never read the contents, we just look for
return codes), so it's not necessary to waste CPU time zeroing on
allocation.

A flame graph analysis showed that we were spending 7% of a xfs_scrub
run (the whole program, not just the attr scrubber itself) allocating
and zeroing 64k segments needlessly.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>

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