This macro is well-suited for use elsewhere in the Rust core.
Move it out to a suitable place, error.rs.

Signed-off-by: Sven Van Asbroeck <thesven73@gmail.com>

This is required to pass the CI.

Signed-off-by: Sven Van Asbroeck <thesven73@gmail.com>

`from_kernel_result!` could panic if the return integer type (`T`) is
unable to hold the negative `errno`. Since `errno`s range from
`1` to `4095`, functions returning integer types unable to hold
values in the [-4095, -1] range could potentially panic.

This includes all unsigned integers, and signed integers with
insufficient bits, such as `c_char`.

Fix by making sure that the return integer type is always suitable
to hold the negative `errno`. Use the Rust type system to verify this
at build time.

Signed-off-by: Sven Van Asbroeck <thesven73@gmail.com>