xfs: introduce new BULKSTAT and INUMBERS ioctls

Now that we've cleaned up the inode walking code, introduce new BULKSTAT
and INUMBERS ioctls.  The new ioctls will report V5 features such as
inode birth time and the ability to do per-AG BULKSTAT and INUMBERS.
The new structures fix the alignment and padding issues that plague the
old ioctls.  In xfsprogs, we will introduce some xfrog_* wrapper
functions that allow new utilities to run on old kernels by emulating
the new functionality.