rgw_file: return of RGWFileHandle::FLAG_EXACT_MATCH

rgw_file:  return of RGWFileHandle::FLAG_EXACT_MATCH

Allow callers of rgw_lookup() on objects attested in an
rgw_readdir() callback the ability to bypass exact match in
RGWLibFS::stat_leaf() case 2, but restore exact match enforcement
for general lookups.

This preserves required common_prefix namespace behavior, but
prevents clients from eerily permitting things like "cd sara0" via
partial name match on "sara01."

Fixes: http://tracker.ceph.com/issues/19059
Signed-off-by: Matt Benjamin <mbenjamin@redhat.com>