jfs: prevent NULL deref in diFree

Add validation check for JFS_IP(ipimap)->i_imap to prevent a NULL deref
in diFree since diFree uses it without do any validations.
When function jfs_mount calls diMount to initialize fileset inode
allocation map, it can fail and JFS_IP(ipimap)->i_imap won't be
initialized. Then it calls diFreeSpecial to close fileset inode allocation
map inode and it will flow into jfs_evict_inode. Function jfs_evict_inode
just validates JFS_SBI(inode->i_sb)->ipimap, then calls diFree. diFree use
JFS_IP(ipimap)->i_imap directly, then it will cause a NULL deref.
Reported-by: NTCS Robot <tcs_robot@tencent.com>
Signed-off-by: NHaimin Zhang <tcs_kernel@tencent.com>
Signed-off-by: NDave Kleikamp <dave.kleikamp@oracle.com>

fs/jfs/inode.c

@@ -146,12 +146,13 @@ void jfs_evict_inode(struct inode *inode)
 		dquot_initialize(inode);
 
 		if (JFS_IP(inode)->fileset == FILESYSTEM_I) {
+			struct inode *ipimap = JFS_SBI(inode->i_sb)->ipimap;
 			truncate_inode_pages_final(&inode->i_data);
 
 			if (test_cflag(COMMIT_Freewmap, inode))
 				jfs_free_zero_link(inode);
 
-			if (JFS_SBI(inode->i_sb)->ipimap)
+			if (ipimap && JFS_IP(ipimap)->i_imap)
 				diFree(inode);
 
 			/*