xfs: introduce xfs_rtalloc_defs.h

There are quite a few realtime device definitions shared with
userspace. Move them from xfs_rtalloc.h to xfs_rt_alloc_defs.h
so we don't need to share xfs_rtalloc.h with userspace anymore.

Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Brian Foster <bfoster@redhat.com>
Reviewed-by: Mark Tinguely <tinguely@sgi.com>
Signed-off-by: Ben Myers <bpm@sgi.com>