AFS: drop explicit extern

Don't use explicit extern specifier and quieten sparse warning:
fs/afs/vnode.c:564:12: warning: function 'afs_vnode_link' with external linkage has definition

Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
Acked-By: David Howells <dhowells@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>