cifs: Fix caching to try to do open O_WRONLY as rdwr on server

When we're engaged in local caching of a cifs filesystem, we cannot perform
caching of a partially written cache granule unless we can read the rest of
the granule.  This can result in unexpected access errors being reported to
the user.

Fix this by the following: if a file is opened O_WRONLY locally, but the
mount was given the "-o fsc" flag, try first opening the remote file with
GENERIC_READ|GENERIC_WRITE and if that returns -EACCES, try dropping the
GENERIC_READ and doing the open again.  If that last succeeds, invalidate
the cache for that file as for O_DIRECT.

Fixes: 70431bfd

 ("cifs: Support fscache indexing rewrite")
Signed-off-by: David Howells <dhowells@redhat.com>
cc: Steve French <sfrench@samba.org>
cc: Shyam Prasad N <nspmangalore@gmail.com>
cc: Rohith Surabattula <rohiths.msft@gmail.com>
cc: Jeff Layton <jlayton@kernel.org>
cc: linux-cifs@vger.kernel.org
cc: netfs@lists.linux.dev
cc: linux-fsdevel@vger.kernel.org
Signed-off-by: Steve French <stfrench@microsoft.com>