osd: abort generate_backlog if already canceled
osd: abort generate_backlog if already canceled

Bail out of generate_backlog if we've been canceled.  Fixes

osd/OSD.cc: In function 'void OSD::generate_backlog(PG*)':
osd/OSD.cc:3305: FAILED assert(!pg->is_active())
 1: ./cosd(_Z18__ceph_assert_failPKcS0_iS0_+0x3a) [0x7a833b]
 2: ./cosd(_ZN3OSD16generate_backlogEP2PG+0xb6) [0x69a1a6]
 3: ./cosd(_ZN3OSD9BacklogWQ8_processEP2PG+0x21) [0x70d92b]
 4: ./cosd(_ZN10ThreadPool9WorkQueueI2PGE13_void_processEPv+0x28) [0x6ca5f8]
 5: ./cosd(_ZN10ThreadPool6workerEv+0x280) [0x7a8efc]
 6: ./cosd(_ZN10ThreadPool10WorkThread5entryEv+0x19) [0x70d331]
 7: ./cosd(_ZN6Thread11_entry_funcEPv+0x20) [0x629e48]
 8: /lib/libpthread.so.0 [0x7f0a8feed3f7]
 9: /lib/libc.so.6(clone+0x6d) [0x7f0a8f4bc94d]

ajnelson/ceph,ajnelson/ceph,ajnelson/ceph,ajnelson/ceph,ajnelson/ceph,ajnelson/ceph