cachefiles: Drop superfluous readpages aops NULL check

commit db58465f upstream.

After the recent actions to convert readpages aops to readahead, the
NULL checks of readpages aops in cachefiles_read_or_alloc_page() may
hit falsely.  More badly, it's an ASSERT() call, and this panics.

Drop the superfluous NULL checks for fixing this regression.

[DH: Note that cachefiles never actually used readpages, so this check was
 never actually necessary]

BugLink: https://bugzilla.kernel.org/show_bug.cgi?id=208883
BugLink: https://bugzilla.opensuse.org/show_bug.cgi?id=1175245
Fixes: 9ae326a6

 ("CacheFiles: A cache that backs onto a mounted filesystem")
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: David Howells <dhowells@redhat.com>
Acked-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>