libxfs: implement some sanity checking for enormous rgcount

libxfs: implement some sanity checking for enormous rgcount

Similar to what we do for suspiciously large sb_agcount values, if
someone tries to get libxfs to load a filesystem with a very large
realtime group count, let's do some basic checks of the rt device to
see if it's really that large.  If the read fails, only load the first
rtgroup and warn the user.

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