nfs: Fix oops in nfs_netfs_init_request() when copying to cache

nfs: Fix oops in nfs_netfs_init_request() when copying to cache

When netfslib wants to copy some data that has just been read on behalf of
nfs, it creates a new write request and calls nfs_netfs_init_request() to
initialise it, but with a NULL file pointer.  This causes
nfs_file_open_context() to oops - however, we don't actually need the nfs
context as we're only going to write to the cache.

Fix this by just returning if we aren't given a file pointer and emit a
warning if the request was for something other than copy-to-cache.

Further, fix nfs_netfs_free_request() so that it doesn't try to free the
context if the pointer is NULL.

Fixes: ee4cdf7ba857 ("netfs: Speed up buffered reading")
Reported-by: Max Kellermann <max.kellermann@ionos.com>
Closes: https://lore.kernel.org/r/CAKPOu+9DyMbKLhyJb7aMLDTb=Fh0T8Teb9sjuf_pze+XWT1VaQ@mail.gmail.com/
Signed-off-by: David Howells <dhowells@redhat.com>
Link: https://lore.kernel.org/r/20241213135013.2964079-5-dhowells@redhat.com
cc: Trond Myklebust <trondmy@kernel.org>
cc: Anna Schumaker <anna@kernel.org>
cc: Dave Wysochanski <dwysocha@redhat.com>
cc: Jeff Layton <jlayton@kernel.org>
cc: linux-nfs@vger.kernel.org
cc: netfs@lists.linux.dev
cc: linux-fsdevel@vger.kernel.org
Signed-off-by: Christian Brauner <brauner@kernel.org>