AFS: Don't pass error value to page_cache_release() in error handling

In the error handling in afs_mntpt_do_automount(), we pass an error
pointer to page_cache_release() if read_mapping_page() failed.  Instead,
we should extend the gotos around the error handling we don't need.

Reported-by: Dan Carpenter <error27@gmail.com>
Signed-off-by: David Howells <dhowells@redhat.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>