cifs: Fix read-performance regression by dropping readahead expansion

cifs_expand_read() is causing a performance regression of around 30% by
causing extra pagecache to be allocated for an inode in the readahead path
before we begin actually dispatching RPC requests, thereby delaying the
actual I/O.  The expansion is sized according to the rsize parameter, which
seems to be 4MiB on my test system; this is a big step up from the first
requests made by the fio test program.

Simple repro (look at read bandwidth number):
     fio --name=writetest --filename=/xfstest.test/foo --time_based --runtime=60 --size=16M --numjobs=1 --rw=read

Fix this by removing cifs_expand_readahead().  Readahead expansion is
mostly useful for when we're using the local cache if the local cache has a
block size greater than PAGE_SIZE, so we can dispense with it when not
caching.

Fixes: 69c3c023

 ("cifs: Implement netfslib hooks")
Signed-off-by: David Howells <dhowells@redhat.com>
Acked-by: Paulo Alcantara (Red Hat) <pc@manguebit.com>
cc: Jeff Layton <jlayton@kernel.org>
cc: Matthew Wilcox <willy@infradead.org>
cc: linux-cifs@vger.kernel.org
cc: netfs@lists.linux.dev
cc: linux-fsdevel@vger.kernel.org
cc: linux-mm@kvack.org
Signed-off-by: Steve French <stfrench@microsoft.com>