xfs: factor out a xfs_rtallocate helper

Split out a helper from xfs_rtallocate that performs the actual
allocation.  This keeps the scope of the xfs_rtalloc_args structure
contained, and prepares for rtgroups support.

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>