fusefs: create sockets with FUSE_MKNOD, not FUSE_CREATE

libfuse expects sockets to be created with FUSE_MKNOD, not FUSE_CREATE,
because that's how Linux does it. My first attempt at creating sockets
(r346894) used FUSE_CREATE because FreeBSD uses VOP_CREATE for this purpose.
There are no backwards-compatibility concerns with this change, because
socket support hasn't yet been merged to head.

Sponsored by: The FreeBSD Foundation