AFS: write back dirty data on unmount

Fix AFS to write back dirty on unmounting.  This didn't happen because
afs_super_ops.drop_inode was pointing to generic_delete_inode.  Now this
pointer is left set to NULL so that the default behaviour occurs instead.

Signed-off-by: David Howells <dhowells@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>