cephfs: support custom replicaiton

Signed-off-by: Noah Watkins <noahwatkins@gmail.com>
cephfs: round down requests for large block sizes

Hadoop uses long for file block size, but the Ceph interface accepts
32-bit integers. Round down and log any instances of block size requests
that are too large.

Signed-off-by: Noah Watkins <noahwatkins@gmail.com>