mon/PGMonitor: fix primary osd check on deep-scrub

s/!=/==/.  Logic was reversed.

Broken in 40bdcb88504aea6288d461d29d24d5b0bf7aeebc.

Signed-off-by: Sage Weil <sage@inktank.com>
Merge pull request #1301 from ceph/wip-scrub-primary

mon/PGMonitor: fix primary osd check on deep-scrub

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