xfs: check rt bitmap file geometry more thoroughly

I forgot that the superblock tracks the number of blocks that are in the
realtime bitmap, and that the rt bitmap file can have more blocks mapped
to the data fork than sb_rbmblocks if growfsrt fails.

So.  Add to the rtbitmap scrubber an explicit check that sb_rextents and
sb_rbmblocks are correct, then adjust the rtbitmap i_size checks to
allow for the growfsrt failure case.  Finally, flag post-eof blocks in
the rtbitmap.

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