[AFS]: Handle multiple mounts of an AFS superblock correctly.

Handle multiple mounts of an AFS superblock correctly, checking to see
whether the superblock is already initialised after calling sget()
rather than just unconditionally stamping all over it.

Also delete the "silent" parameter to afs_fill_super() as it's not
used and can, in any case, be obtained from sb->s_flags.

Signed-off-by: David Howells <dhowells@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>