mds: add maximum fragment size constraint

mds: add maximum fragment size constraint

This commit adds a new config option

    mds_bal_fragment_size_max = 10000*10

which is an order of magnitude larger than mds_bal_split_size.

This limit prevents a fragment from getting too large which results in large
omap directories.

Right now the limit is enforced only in the RPC paths and in stray directory
entry creation.

Fixes http://tracker.ceph.com/issues/16164

Signed-off-by: Patrick Donnelly <pdonnell@redhat.com>