xfs: check for dinode realtime flag corruption

xfs: check for dinode realtime flag corruption

Ramon tested XFS with a modified version of fsfuzzer and hit a NULL
pointer dereference in __xfs_get_blocks due to the RT device target
pointer being NULL.

To fix this reject inode with the realtime bit set on a a filesystem
without an RT subvolume during inode read.

Signed-off-by: Christoph Hellwig <[email protected]>
Reviewed-by: Eric Sandeen <[email protected]>
Reviewed-by: Felix Blyakher <[email protected]>
Reported-by: Ramon de Carvalho Valle <[email protected]>
Tested-by: Ramon de Carvalho Valle <[email protected]>
Signed-off-by: Felix Blyakher <[email protected]>