fuse: drop unused fuse_mount arg in fuse_writepage_finish()

Drop the unused "struct fuse_mount *fm" arg in
fuse_writepage_finish().

No functional changes added.

Signed-off-by: Joanne Koong <joannelkoong@gmail.com>
Reviewed-by: Jingbo Xu <jefflexu@linux.alibaba.com>
Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>