xfs: split xfs_rtalloc.c for userspace sanity

xfs_rtalloc.c is partially shared with userspace. Split the file up
into two parts - one that is kernel private and the other which is
wholly shared with userspace.

Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Ben Myers <bpm@sgi.com>