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

[ Upstream commit 4e9036042fedaffcd868d7f7aa948756c48c637d ]

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>
Signed-off-by: Sasha Levin <sashal@kernel.org>