ext4: add EXT4_IGET_BAD flag to prevent unexpected bad inode

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

ext4: add EXT4_IGET_BAD flag to prevent unexpected bad inode

There are many places that will get unhappy (and crash) when ext4_iget()

returns a bad inode. However, if iget the boot loader inode, allows a bad

inode to be returned, because the inode may not be initialized. This

mechanism can be used to bypass some checks and cause panic. To solve this

problem, we add a special iget flag EXT4_IGET_BAD. Only with this flag

we'd be returning bad inode from ext4_iget(), otherwise we always return

the error code if the inode is bad inode.(suggested by Jan Kara)