afs: Fix RCU handling of symlinks in RCU pathwalk

The afs filesystem in the kernel doesn't handle RCU pathwalk of symlinks
correctly.  The problem is twofold: firstly, it doesn't treat the buffer
pointers as RCU pointers with the appropriate barriering; and secondly, it
can race with another thread updating the contents of the symlink because a
third party updated it on the server.

Fix this by the following means:

 (1) Keep a separate copy of the symlink contents with an rcu_head.  This
     is always going to be a lot smaller than a page, so it can be
     kmalloc'd and save quite a bit of memory.  It also needs a refcount
     for non-RCU pathwalk.

 (2) Split the symlink read and write-to-cache routines in afs from those
     for directories.

 (3) Discard the I/O buffer as soon as the write-to-cache completes as this
     is a full page (plus a folio_queue).

 (4) If there's no cache, discard the I/O buffer immediately after reading
     and copying if there is no cache.

Fixes: 6698c02d64b2 ("afs: Locally initialise the contents of a new symlink on creation")
Closes: https://sashiko.dev/#/patchset/20260326104544.509518-1-dhowells%40redhat.com
Signed-off-by: David Howells <dhowells@redhat.com>
cc: Marc Dionne <marc.dionne@auristor.com>
cc: linux-afs@lists.infradead.org
cc: linux-fsdevel@vger.kernel.org