fuse: Fix GID when sending FUSE_INTERRUPT to a FUSE daemon

Due to the partial-only changes of commit 46c07316f9 ("kern: adopt the
cr_gid macro for cr_groups[0] more widely"), subsequent commit
be1f7435ef ("kern: start tracking cr_gid outside of cr_groups[]")
caused a mismatch between filling cr_groups[0] in 'reused_creds' in
fuse_interrupt_send() and reading 'cr_gid' from it in
fuse_setup_ihead(), with the consequence that the kernel would send
a FUSE_INTERRUPT message to the FUSE deamon with an uninitialized GID in
its header (which, besides being wrong, would disclose 4 bytes from its
stack).

Fixes:          be1f7435ef ("kern: start tracking cr_gid outside of cr_groups[]")
MFC after:      5 days
MFC to:         stable/15
Sponsored by:   The FreeBSD Foundation
Differential Revision:  https://reviews.freebsd.org/D52255

sys/fs/fuse/fuse_ipc.c

@@ -193,7 +193,6 @@ fuse_interrupt_send(struct fuse_ticket *otick, int err)
 	struct fuse_data *data = otick->tk_data;
 	struct fuse_ticket *tick, *xtick;
 	struct ucred reused_creds;
-	gid_t reused_groups[1];
 
 	if (otick->irq_unique == 0) {
 		/* 
@@ -237,8 +236,7 @@ fuse_interrupt_send(struct fuse_ticket *otick, int err)
 		 */
 		ftick_hdr = fticket_in_header(otick);
 		reused_creds.cr_uid = ftick_hdr->uid;
-		reused_groups[0] = ftick_hdr->gid;
-		reused_creds.cr_groups = reused_groups;
+		reused_creds.cr_gid = ftick_hdr->gid;
 		fdisp_init(&fdi, sizeof(*fii));
 		fdisp_make_pid(&fdi, FUSE_INTERRUPT, data, ftick_hdr->nodeid,
 			ftick_hdr->pid, &reused_creds);