cramfs: generate unique inode number for better inode cache usage

Generate a unique inode numbers for any entries in the cram file system.
For files which did not contain data's (device nodes, fifos and sockets)
the offset of the directory entry inside the cramfs plus 1 will be used as
inode number.

The + 1 for the inode will it make possible to distinguish between a file
which contains no data and files which has data, the later one has a inode
value where the lower two bits are always 0.

It also reimplements the behavior to set the size and the number of block
to 0 for special file, which is the right value for empty files, devices,
fifos and sockets

As a little benefit it will be also more compatible which older mkcramfs,
because it will never use the cramfs_inode->offset for creating a inode
number for special files.

[akpm@linux-foundation.org: trivial comment fix]
[akpm@linux-foundation.org: coding-style fixes]
Signed-off-by: Stefani Seibold <stefani@seibold.net>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Pekka Enberg <penberg@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>