freevxfs: implement ->alloc_inode and ->destroy_inode

This driver predates those methods and was trying to be clever allocating
it's own private data.  Switch to the generic scheme used by other file
systems.

Based on an earlier patch from Krzysztof Błaszkowski <kb@sysmikro.com.pl>.

Signed-off-by: Christoph Hellwig <hch@lst.de>