fuse: ->direct_IO() doesn't need generic_write_checks()

already done by caller.  We used to call __fuse_direct_write(), which
called generic_write_checks(); now the former got expanded, bringing
the latter to the surface.  It used to be called all along and calling
it from there had been wrong all along...

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>