Merge pull request #7957 from majianpeng/bluestore-read

os/filestore: require offset == length == 0 for full object read; add test

Reviewed-by: Sage Weil <sage@redhat.com>
Reviewed-by: Igor Fedotov <ifedotov@mirantis.com>

diff --cc src/os/filestore/FileStore.cc
Simple merge

diff --cc src/test/objectstore/store_test.cc
Simple merge