rust: error: Add a helper to convert a C ERR_PTR to a `Result`

Some kernel C API functions return a pointer which embeds an optional
`errno`. Callers are supposed to check the returned pointer with
`IS_ERR()` and if this returns `true`, retrieve the `errno` using
`PTR_ERR()`.

Create a Rust helper function to implement the Rust equivalent:
transform a `*mut T` to `Result<*mut T>`.

Lina: Imported from rust-for-linux/linux, with subsequent refactoring
and contributions squashed in and attributed below. Renamed the function
to from_err_ptr().

Co-developed-by: Boqun Feng <[email protected]>
Signed-off-by: Boqun Feng <[email protected]>
Co-developed-by: Miguel Ojeda <[email protected]>
Signed-off-by: Miguel Ojeda <[email protected]>
Co-developed-by: Fox Chen <[email protected]>
Signed-off-by: Fox Chen <[email protected]>
Co-developed-by: Gary Guo <[email protected]>
Signed-off-by: Gary Guo <[email protected]>
Signed-off-by: Sven Van Asbroeck <[email protected]>
Reviewed-by: Martin Rodriguez Reboredo <[email protected]>
Signed-off-by: Asahi Lina <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
[ Add a removal of `#[allow(dead_code)]`. ]
Signed-off-by: Miguel Ojeda <[email protected]>

0 files changed, 0 insertions, 0 deletions