virtiofsd: Parse extended "struct fuse_init_in"

virtiofsd: Parse extended "struct fuse_init_in"

Add some code to parse extended "struct fuse_init_in". And use a local
variable "flag" to represent 64 bit flags. This will make it easier
to add more features without having to worry about two 32bit flags (->flags
and ->flags2) in "fuse_struct_in".

Signed-off-by: Vivek Goyal <vgoyal@redhat.com>
Message-Id: <20220208204813.682906-4-vgoyal@redhat.com>
Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
  dgilbert: Fixed up long line