jfs: fix GPF in diFree

In the Linux kernel, the following vulnerability has been resolved:

jfs: fix GPF in diFree

Avoid passing inode with
JFS_SBI(inode->i_sb)->ipimap == NULL to
diFree()[1]. GFP will appear:

struct inode *ipimap = JFS_SBI(ip->i_sb)->ipimap;
struct inomap *imap = JFS_IP(ipimap)->i_imap;

JFS_IP() will return invalid pointer when ipimap == NULL

Call Trace:
diFree+0x13d/0x2dc0 fs/jfs/jfs_imap.c:853 [1]
jfs_evict_inode+0x2c9/0x370 fs/jfs/inode.c:154
evict+0x2ed/0x750 fs/inode.c:578
iput_final fs/inode.c:1654 [inline]
iput.part.0+0x3fe/0x820 fs/inode.c:1680
iput+0x58/0x70 fs/inode.c:1670