hfsplus: initialise userflags

The userflags field was being written to the filesystem without being
initialised. Make sure it's clear, since otherwise files end up with
garbage attributes.

Signed-off-by: Matthew Garrett <mjg@redhat.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>