xfs: factor out a xfs_growfs_rt_bmblock helper

xfs: factor out a xfs_growfs_rt_bmblock helper

Add a helper to contain the per-rtbitmap block logic in xfs_growfs_rt.

Note that this helper now allocates a new fake mount structure for
each rtbitmap block iteration instead of reusing the memory for an
entire growfs call.  Compared to all the other work done when freeing
the blocks the overhead for this is in the noise and it keeps the code
nicely modular.

Signed-off-by: Christoph Hellwig <hch@lst.de>