Coverity fixes in fusefs(5)

CID 1404532 fixes a signed vs unsigned comparison error in fuse_vnop_bmap.
It could potentially have resulted in VOP_BMAP reporting too many
consecutive blocks.

CID 1404364 is much worse. It was an array access by an untrusted,
user-provided variable. It could potentially have resulted in a malicious
file system crashing the kernel or worse.

Reported by: Coverity
Reviewed by: emaste
MFC after: 3 days
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D21466