fuse: update inode size after extending passthrough write

yangyun reported that libfuse test test_copy_file_range() copies zero
bytes from a newly written file when fuse passthrough is enabled.

The reason is that extending passthrough write is not updating the fuse
inode size and when vfs_copy_file_range() observes a zero size inode,
it returns without calling the filesystem copy_file_range() method.

Fix this by adjusting the fuse inode size after an extending passthrough
write.

This does not provide cache coherency of fuse inode attributes and
backing inode attributes, but it should prevent situations where fuse
inode size is too small, causing read/copy to be wrongly shortened.

Reported-by: yangyun <yangyun50@huawei.com>
Closes: https://github.com/libfuse/libfuse/issues/1048


Fixes: 57e1176e ("fuse: implement read/write passthrough")
Signed-off-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>