Fix for JFS handling of device nodes; it has 32bit on-disk device
numbers, shoves them into 16bit (->i_rdev) when inode is read and writes
them back truncated when inode is written to disk.  For now (and 2.4 will
have to do the same permanently) we store the original value in private
part of inode and use it instead of ->i_rdev in ->write_inode(); mknod()
sets it at the same time as ->i_rdev.  It will become unnecessary when
dev_t becomes wider than 16 bits, but for now we need it.