hfs: Call hfs_write_begin() and generic_write_end() directly

There is only one kind of write_begin/write_end aops, so we don't need
to look up which aop it is, just make hfs_write_begin() available to
this file and call it directly.

Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>