netfs: Fix writethrough-mode error handling

Fix the error return in netfs_perform_write() acting in writethrough-mode
to return any cached error in the case that netfs_end_writethrough()
returns 0.

This can affect the use of O_SYNC/O_DSYNC/RWF_SYNC/RWF_DSYNC in 9p and afs.

Fixes: 41d8e767 ("netfs: Implement a write-through caching option")
Signed-off-by: David Howells <dhowells@redhat.com>
Link: https://lore.kernel.org/r/6736.1713343639@warthog.procyon.org.uk


Reviewed-by: Jeff Layton <jlayton@kernel.org>
cc: Eric Van Hensbergen <ericvh@kernel.org>
cc: Latchesar Ionkov <lucho@ionkov.net>
cc: Dominique Martinet <asmadeus@codewreck.org>
cc: Christian Schoenebeck <linux_oss@crudebyte.com>
cc: Marc Dionne <marc.dionne@auristor.com>
cc: netfs@lists.linux.dev
cc: linux-fsdevel@vger.kernel.org
cc: v9fs@lists.linux.dev
cc: linux-afs@lists.infradead.org
Signed-off-by: Christian Brauner <brauner@kernel.org>