iget: remove iget() and the read_inode() super op as being obsolete

Remove the old iget() call and the read_inode() superblock operation it uses
as these are really obsolete, and the use of read_inode() does not produce
proper error handling (no distinction between ENOMEM and EIO when marking an
inode bad).

Furthermore, this removes the temptation to use iget() to find an inode by
number in a filesystem from code outside that filesystem.

iget_locked() should be used instead.  A new function is added in an earlier
patch (iget_failed) that is to be called to mark an inode as bad, unlock it
and release it should the get routine fail.  Mark iget() and read_inode() as
being obsolete and remove references to them from the documentation.

Typically a filesystem will be modified such that the read_inode function
becomes an internal iget function, for example the following:

	void thingyfs_read_inode(struct inode *inode)
	{
		...
	}

would be changed into something like:

	struct inode *thingyfs_iget(struct supe...