fs/9p: fix uninitialized values during inode evict

If an iget fails due to not being able to retrieve information
from the server then the inode structure is only partially
initialized.  When the inode gets evicted, references to
uninitialized structures (like fscache cookies) were being
made.

This patch checks for a bad_inode before doing anything other
than clearing the inode from the cache.  Since the inode is
bad, it shouldn't have any state associated with it that needs
to be written back (and there really isn't a way to complete
those anyways).

Reported-by:  <syzbot+eb83fe1cce5833cd66a0@syzkaller.appspotmail.com>
Signed-off-by: Eric Van Hensbergen <ericvh@kernel.org>