xfs: factor out a xfs_growfs_rt_alloc_blocks helper

Split out a helper to allocate or grow the rtbitmap and rtsummary files
in preparation of per-RT group bitmap and summary files.

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