fuse: fix VM_MAYSHARE and direct_io_allow_mmap

fuse: fix VM_MAYSHARE and direct_io_allow_mmap

There were multiple issues with direct_io_allow_mmap:

 - fuse_link_write_file() was missing, resulting in warnings in
   fuse_write_file_get() and EIO from msync()

 - "vma->vm_ops = &fuse_file_vm_ops" was not set, but especially
   fuse_page_mkwrite is needed.

The semantics of invalidate_inode_pages2() is so far not clearly defined in
fuse_file_mmap.  It dates back to commit 3121bfe76311 ("fuse: fix
"direct_io" private mmap") Though, as direct_io_allow_mmap is a new
feature, that was for MAP_PRIVATE only.  As invalidate_inode_pages2() is
calling into fuse_launder_folio() and writes out dirty pages, it should be
safe to call invalidate_inode_pages2 for MAP_PRIVATE and MAP_SHARED as
well.

Cc: Hao Xu <howeyxu@tencent.com>
Cc: stable@vger.kernel.org
Fixes: e78662e818f9 ("fuse: add a new fuse init flag to relax restrictions in no cache mode")
Signed-off-by: Bernd Schubert <bschubert@ddn.com>
Reviewed-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>