cephfs: align file block size to stripe unit

Forces new files created through the Hadoop VFS to have block sizes that
are aligned (rounded up) to the Ceph stripe unit granularity.

Signed-off-by: Joe Buck <jbbuck@gmail.com>
Signed-off-by: Noah Watkins <noahwatkins@gmail.com>
cephfs: use named constant for default stripe count

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