ufs: fix braino in ufs_get_inode_gid() for solaris UFS flavour

To choose whether to pick the GID from the old (16bit) or new (32bit)
field, we should check if the old gid field is set to 0xffff.  Mainline
checks the old *UID* field instead - cut'n'paste from the corresponding
code in ufs_get_inode_uid().

Fixes: 252e211e
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>