fuse: require /dev/fuse reads to have enough buffer capacity

A FUSE filesystem server queues /dev/fuse sys_read calls to get
filesystem requests to handle. It does not know in advance what would be
that request as it can be anything that client issues - LOOKUP, READ,
WRITE, ... Many requests are short and retrieve data from the
filesystem. However WRITE and NOTIFY_REPLY write data into filesystem.

Before getting into operation phase, FUSE filesystem server and kernel
client negotiate what should be the maximum write size the client will
ever issue. After negotiation the contract in between server/client is
that the filesystem server then should queue /dev/fuse sys_read calls with
enough buffer capacity to receive any client request - WRITE in
particular, while FUSE client should not, in particular, send WRITE
requests with > negotiated max_write payload. FUSE client in kernel and
libfuse historically reserve 4K for request header. This way the
contract is that filesystem server should queue sys_reads with
4K+max_write buffer.

If the filesystem server does not follow this contract, what can happen
is that fuse_dev_do_read will see that request size is > buffer size,
and then it will return EIO to client who issued the request but won't
indicate in any way that there is a problem to filesystem server.
This can be hard to diagnose because for some requests, e.g. for
NOTIFY_REPLY which mimics WRITE, there is no client thread that is
waiting for request completion and that EIO goes nowhere, while on
filesystem server side things look like the kernel is not replying back
after successful NOTIFY_RETRIEVE request made by the server.

We can make the problem easy to diagnose if we indicate via error return to
filesystem server when it is violating the contract.  This should not
practically cause problems because if a filesystem server is using shorter
buffer, writes to it were already very likely to cause EIO, and if the
filesystem is read-only it should be too following FUSE_MIN_READ_BUFFER
minimum buffer size.

Please see [1] for context where the problem of stuck filesystem was hit
for real (because kernel client was incorrectly sending more than
max_write data with NOTIFY_REPLY; see also previous patch), how the
situation was traced and for more involving patch that did not make it
into the tree.

[1] https://marc.info/?l=linux-fsdevel&m=155057023600853&w=2

Signed-off-by: Kirill Smelkov <kirr@nexedi.com>
Cc: Han-Wen Nienhuys <hanwen@google.com>
Cc: Jakob Unterwurzacher <jakobunt@gmail.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>

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