os/,osd/: restructure the rados name length check

os/,osd/: restructure the rados name length check

Enforce locator length vs the max name length and also
introduce a namespace length limit.

In addition to these checks, also pass the head object to the
ObjectStore implementation to validate.  This allows LFNIndex to account
for the idiosyncracies of its filename escaping and for different xattr
value max sizes.

Signed-off-by: Samuel Just <sjust@redhat.com>