fuse: Allocate only namelen buf memory in fuse_notify_

fuse_notify_inval_entry and fuse_notify_delete were using fixed allocations
of FUSE_NAME_MAX to hold the file name. Often that large buffers are not
needed as file names might be smaller, so this uses the actual file name
size to do the allocation.

Signed-off-by: Bernd Schubert <bschubert@ddn.com>
Reviewed-by: Jingbo Xu <jefflexu@linux.alibaba.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>