xfs: make the RT rsum_cache mandatory

Currently the RT mount code simply ignores an allocation failure for the
rsum_cache.  The code mostly works fine with it, but not having it leads
to nasty corner cases in the growfs code that we don't really handle
well.  Switch to failing the mount if we can't allocate the memory, the
file system would not exactly be useful in such a constrained environment
to start with.

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>