freevxfs: handle big endian HP-UX file systems

To support VxFS filesystems from HP-UX on x86 systems we need to
implement byte swapping, and to keep support for Unixware filesystems
it needs to be the complicated dual-endian kind ala sysvfs.

To do this properly we have to split the on disk and in-core inode
so that we can keep the in-core one in native endianness.  All other
structures are byteswapped on demand.

Signed-off-by: Krzysztof Błaszkowski <kb@sysmikro.com.pl>
[hch: make spare happy]
Signed-off-by: Christoph Hellwig <hch@lst.de>