fusefs: diff reduction vs the upstream sources

fuse_kernel.h defines the structures used by the FUSE protocol. Originally
it came from libfuse, but the current source of truth is the Linux kernel.
This commit minimizes the diffs between our version and the Linux version as
of 21f3da95d (protocol version 7.8).

The flags field of struct fuse_listxattr_out and fuse_listxattr_in was an
error in our header. Those fields don't exist in Linux or libfuse, and
they've never been used in FreeBSD. In fact, those structs don't even exist
in Linux and libfuse; those projects confusingly overload the identical
fuse_getexattr_in and fuse_getxattr_out structs.

Sponsored by: The FreeBSD Foundation