rust: remove unnecessary parentheses (Rust 1.40 fixup)

rust: remove unnecessary parentheses (Rust 1.40 fixup)

Rust 1.40 in strict mode will now fail the build on the
presence of unnecessary parentheses.

warning: unnecessary parentheses around type
  --> src/smb/smb2_ioctl.rs:41:12
   |
41 |         -> (&mut SMBTransaction)
   |            ^^^^^^^^^^^^^^^^^^^^^ help: remove these parentheses
   |
   = note: `#[warn(unused_parens)]` on by default

(cherry picked from commit 5ee8323028aeb0d2c4926bae3bf8dafd05215d56)