PGLog::rewind_divergent_log: unindex only works from tail, index() instead

Fixes: #5714
Signed-off-by: Samuel Just <sam.just@inktank.com>
Reviewed-by: Sage Weil <sage@inktank.com>
(cherry picked from commit 6957dbc75cc2577652b542aa3eae69f03060cb63)

The original patch covered the same code in PGLog.cc.

Conflicts:

	src/osd/PGLog.cc
	src/osd/PG.cc
FileStore::_collection_rename: fix global replay guard

If the replay is being replayed, we might have already
performed the rename, skip it.  Also, we must set the
collection replay guard only after we have done the
rename.

Signed-off-by: Samuel Just <sam.just@inktank.com>