AFS: Stop readlink() on AFS crashing due to NULL 'file' ptr

kAFS crashes when asked to read a symbolic link because page_getlink()
passes a NULL file pointer to read_mapping_page(), but afs_readpage()
expects a file pointer from which to extract a key.

Modify afs_readpage() to request the appropriate key from the calling
process's keyrings if a file struct is not supplied with one attached.

Signed-off-by: David Howells <dhowells@redhat.com>
Acked-by: Anton Blanchard <anton@samba.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>