xfs: check rt summary file geometry more thoroughly

I forgot that the xfs_mount tracks the size and number of levels in the
realtime summary file, and that the rt summary file can have more blocks
mapped to the data fork than m_rsumsize implies if growfsrt fails.

So.  Add to the rtsummary scrubber an explicit check that all the
summary geometry values are correct, then adjust the rtsummary i_size
checks to allow for the growfsrt failure case.  Finally, flag post-eof
blocks in the summary file.

While we're at it, split the extent map checking so that we only call
xfs_bmapi_read once per extent instead of once per rtsummary block.

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