afs: Fix dynamic root getattr

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

afs: Fix dynamic root getattr

The recent patch to make afs_getattr consult the server didn't account
for the pseudo-inodes employed by the dynamic root-type afs superblock
not having a volume or a server to access, and thus an oops occurs if
such a directory is stat'd.

Fix this by checking to see if the vnode->volume pointer actually points
anywhere before following it in afs_getattr().

This can be tested by stat'ing a directory in /afs. It may be
sufficient just to do "ls /afs" and the oops looks something like:

BUG: kernel NULL pointer dereference, address: 0000000000000020
...
RIP: 0010:afs_getattr+0x8b/0x14b
...
Call Trace:
<TASK>
vfs_statx+0x79/0xf5
vfs_fstatat+0x49/0x62