Patch from Andries.Brouwer@cwi.nl

The next patch in the dev_t series eliminates the last applied use
of MAX_BLKDEV - only the definition in major.h remains.

Sneaky as I am, I combine this patch with the fix for an Oops:
On open, raw_open does
	filp->f_dentry->d_inode->i_mapping =
		bdev->bd_inode->i_mapping;
storing a pointer to bdev stuff.
But on release this pointer stayed, the block device is not
referenced anymore and disappears, and the next open references
undefined stuff.
I checked, and this can actually cause an Oops - scenario:

  # raw /dev/raw/raw12 /dev/hdf
  # dd if=/dev/raw/raw12 of=/dev/null bs=512 count=1
  # raw /dev/raw/raw12 0 0
  # dd if=/dev/raw/raw12 of=/dev/null bs=512 count=1

Oops.

More precisely the problem is that dentry_open does
file_ra_state_init(&f->f_ra, inode->i_mapping);
And file_ra_state_init uses mapping->backing_dev_info->ra_pages.
Ugly, to use so much information about the inode even before
the inode has been opened.

In the patch below I reset i_mapping upon release of the raw device.

akpm: I fixed a typo and exported default_backing_dev_info to GPL modules for
this.