xfs: create routine to allocate and initialize a realtime rmap btree inode

xfs: create routine to allocate and initialize a realtime rmap btree inode

Create a library routine to allocate and initialize an empty realtime
rmapbt inode.  We'll use this for mkfs and repair.

Signed-off-by: "Darrick J. Wong" <djwong@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>