mds: init inode_t::dir_layout

mds: init inode_t::dir_layout

At (2): Non-static class member field "dir_layout.dl_dir_hash" is not initialized in this constructor nor in any functions that it calls.
At (4): Non-static class member field "dir_layout.dl_unused1" is not initialized in this constructor nor in any functions that it calls.
At (6): Non-static class member field "dir_layout.dl_unused2" is not initialized in this constructor nor in any functions that it calls.
CID 717258 (#1 of 1): Uninitialized scalar field (UNINIT_CTOR)

Signed-off-by: Sage Weil <sage@inktank.com>