xfs: support creating per-RTG files in growfs

xfs: support creating per-RTG files in growfs

To support adding new RT groups in growfs, we need to be able to create
the per-RT group files.  Add a new xfs_rtginode_create helper to create
a given per-RTG file.  Most of the code for that is shared, but the
details of the actual file are abstracted out using a new create method
in struct xfs_rtginode_ops.

[Note: this could probably move significantly later in the series]
Signed-off-by: Christoph Hellwig <hch@lst.de>