fusefs: ensure that FUSE ops' headers' unique values are actually unique

Every FUSE operation has a unique value in its header. As the name
implies, these values are supposed to be unique among all outstanding
operations. And since FUSE_INTERRUPT is asynchronous and racy, it is
desirable that the unique values be unique among all operations that are
"close in time".

Ensure that they are actually unique by incrementing them whenever we
reuse a fuse_dispatcher object, for example during fsync, write, and
listextattr.

PR: 244686
MFC after: 2 weeks
Reviewed by: pfg
Differential Revision: https://reviews.freebsd.org/D30810