afs: Use new fscache read helper API

afs: Use new fscache read helper API

Make AFS use the new fscache read helpers to implement the VM read
operations:

 - afs_readpage() now hands off responsibility to fscache_readpage().

 - afs_readpages() is gone and replaced with afs_readahead().

 - afs_readahead() just hands off responsibility to fscache_readahead().

These make use of the cache if a cookie is supplied, otherwise just call
the ->issue_op() method a sufficient number of times to complete the entire
request.

Signed-off-by: David Howells <dhowells@redhat.com>