mon/OSDMonitor: fix osd epoch in boot check

This was introduced in 4c99e978a77a242e540cb8ccacb967d24322416c and was
incorrect; boot_epoch is the previous epoch the osd booted in, not the
latest map epoch that the OSD currently has.

Signed-off-by: Sage Weil <sage@inktank.com>
Merge pull request #1656 from ceph/wip-osd-boot

mon: fix osd boot check

Reviewed-by: Greg Farnum <greg@inktank.com>