fusefs: replace the fufh table with a linked list

fusefs: replace the fufh table with a linked list

The FUSE protocol allows each open file descriptor to have a unique file
handle.  On FreeBSD, these file handles must all be stored in the vnode.
The old method (also used by OSX and OpenBSD) is to store them all in a
small array.  But that limits the total number that can be stored.  This
commit replaces the array with a linked list (a technique also used by
Illumos).  There is not yet any change in functionality, but this is the
first step to fixing several bugs.

PR:		236329, 236340, 236381, 236560, 236844
Discussed with:	cem
Sponsored by:	The FreeBSD Foundation