afs: Fix accidental truncation when storing data

When an AFS FS.StoreData RPC call is made, amongst other things it is
given the resultant file size to be. On the server, this is processed
by truncating the file to new size and then writing the data.

Now, kafs has a lock (vnode->io_lock) that serves to serialise
operations against a specific vnode (ie. inode), but the parameters for
the op are set before the lock is taken. This allows two writebacks
(say sync and kswapd) to race - and if writes are ongoing the writeback
for a later write could occur before the writeback for an earlier one if
the latter gets interrupted.

Note that afs_writepages() cannot take i_mutex and only takes a shared
lock on vnode->validate_lock.

Also note that the server does the truncation and the write inside a
lock, so there's no problem at that end.

Fix this by moving the calculation for the proposed new i_size inside
the vnode->io_lock. Also reset the iterator (which we might have read
from) and update the mtime setting there.

Fixes: bd80d8a80e12 ("afs: Use ITER_XARRAY for writing")
Reported-by: Marc Dionne <marc.dionne@auristor.com>
Signed-off-by: David Howells <dhowells@redhat.com>
Reviewed-by: Jeffrey Altman <jaltman@auristor.com>
Reviewed-by: Marc Dionne <marc.dionne@auristor.com>
cc: linux-afs@lists.infradead.org
cc: linux-fsdevel@vger.kernel.org
Link: https://lore.kernel.org/r/3526895.1687960024@warthog.procyon.org.uk/
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>