fusefs: Handle ENOSYS for all remaining opcodes

For many FUSE opcodes, an error of ENOSYS has special meaning. fusefs
already handled some of those; this commit adds handling for the remainder:

• FUSE_FSYNC, FUSE_FSYNCDIR: ENOSYS means "success, and automatically return success without calling the daemon from now on"
• All extattr operations: ENOSYS means "fail EOPNOTSUPP, and automatically do it without calling the daemon from now on"

PR: 236557
Sponsored by: The FreeBSD Foundation