fusefs: implement VOP_BMAP

If the fuse daemon supports FUSE_BMAP, then use that for the block mapping.
Otherwise, use the same technique used by vop_stdbmap. Report large values
for runp and runb in order to maximize read clustering and minimize upcalls,
even if we don't know the true layout.

The major result of this change is that sequential reads to FUSE files will
now usually happen 128KB at a time instead of 64KB.

Sponsored by: The FreeBSD Foundation