osd: make add_next_entry behave when we start at backlog split point
osd: make add_next_entry behave when we start at backlog split point

Weaken the assertions a bit and just adjust missing appropriately.
Things may not match up perfectly if the split point is a backlog
entry, so just make missing what it should be a worry less about
what it was.

Here is the specific crash:

09.06.18 16:29:15.085353 1124096336 osd1 10 pg[1.8( v 5'4/3'2 (0'0,5'4] n=2 ec=2 les=10 10/3) r=1 lcod 0'0 stray m=1] my log = log(0'0,5'4]+backlog
3'1 (0'0) m 200.00000000/head by mds0.1:1 09.06.18 16:20:07.524996 indexed
3'2 (0'0) m 2.00000000/head by mds0.1:5 09.06.18 16:20:07.527454 indexed
5'3 (3'1) m 200.00000000/head by mds0.1:23 09.06.18 16:20:25.128842 indexed
5'4 (5'3) m 200.00000000/head by mds0.1:35 09.06.18 16:20:48.623669 indexed

09.06.18 16:29:15.085393 1124096336 osd1 10 pg[1.8( v 5'4/3'2 (0'0,5'4] n=2 ec=2 les=10 10/3) r=1 lcod 0'0 stray m=1] osd2 log = log(8'68,9'69]+backlog
3'2 (0'0) b 2.00000000/head by mds0.1:5 09.06.18 16:20:07.527454
9'69 (8'68) m 200.00000000/head by mds0.1:1114 09.06.18 16:28:08.837907

09.06.18 16:29:15.085416 1124096336 osd1 10 pg[1.8( v 5'4/3'2 (0'0,5'4] n=2 ec=2 les=10 10/3) r=1 lcod 0'0 stray m=1] merge_log log(8'68,9'69]+backlog from osd2 into log(0'0,5'4]+backlog
09.06.18 16:29:15.085456 1124096336 osd1 10 pg[1.8( v 5'4/3'2 (0'0,5'4] n=2 ec=2 les=10 10/3) r=1 (log bound mismatch, actual=[3'2,9'69] len=2) lcod 0'0 stray m=1] merge_log split point is 3'2 (0'0) b 2.00000000/head by mds0.1:5 09.06.18 16:20:07.527454
09.06.18 16:29:15.085472 1124096336 osd1 10 pg[1.8( v 5'4/3'2 (0'0,5'4] n=2 ec=2 les=10 10/3) r=1 (log bound mismatch, actual=[3'2,9'69] len=2) lcod 0'0 stray m=1] merge_log merging 3'2 (0'0) b 2.00000000/head by mds0.1:5 09.06.18 16:20:07.527454
09.06.18 16:29:15.085493 1124096336 osd1 10 pg[1.8( v 5'4/3'2 (0'0,5'4] n=2 ec=2 les=10 10/3) r=1 (log bound mismatch, actual=[3'2,9'69] len=2) lcod 0'0 stray m=2] merge_log merging 9'69 (8'68) m 200.00000000/head by mds0.1:1114 09.06.18 16:28:08.837907
osd/PG.h: In function 'void PG::Missing::add_next_event(PG::Log::Entry&)':
osd/PG.h:494: FAILED assert(missing[e.soid].need == e.prior_version)

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