Use a zero sized array for raw field in struct fid

The raw field's size can vary so we use a zero sized array since
gcc will not allow a variable sized array inside a union. This
has been tested with ext3 and gfs2 and relates to the bug
report: http://lkml.org/lkml/2007/10/24/374 and discussion
thread: http://lkml.org/lkml/2008/4/7/65



Signed-off-by: Steven Whitehouse <swhiteho@redhat.com>
Cc: Christoph Hellwig <hch@infradead.org>
Cc: Neil Brown <neilb@suse.de>
Cc: Adrian Bunk <bunk@kernel.org>
Signed-off-by: J. Bruce Fields <bfields@citi.umich.edu>