osd/ReplicatedPG: fix bl resize on write vs truncate race

If we resize the write due to the funky truncate behavior, we need to
resize the bufferlist to match.

Fixes: #6346
Signed-off-by: Sage Weil <sage@inktank.com>
Merge pull request #645 from liewegas/wip-6346

Reviewed-by: Greg Farnum <greg@inktank.com>