os/bluestore: handle spurious read errors

os/bluestore: handle spurious read errors

Some kernels (4.9+) sometime fail to return data when reading
from a block device under memory pressure. This patch retries
the read if the checksum verification fails, tests show that
the first retried read succeeds in ~99.5% of the cases, so
3 attempts are made by default before giving up on the data.

Works-around: http://tracker.ceph.com/issues/22464
Signed-off-by: Paul Emmerich <paul.emmerich@croit.io>
(cherry picked from commit cffcbc73aaaa874829d5fc9091af3042b887f9a7)

- conflict due to adjacent tests in store_test
- g_conf, not g_conf()