rust: uaccess: add UserSliceReader::strcpy_into_buf

This patch adds a more convenient method for reading C strings from
userspace. Logic is added to NUL-terminate the buffer when necessary so
that a &CStr can be returned.

Note that we treat attempts to read past `self.length` as a fault, so
this returns EFAULT if that limit is exceeded before `buf.len()` is
reached.

Reviewed-by: Danilo Krummrich <[email protected]>
Signed-off-by: Alice Ryhl <[email protected]>
Reviewed-by: Benno Lossin <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
[ Use `from_mut` to clean `clippy::ref_as_ptr` lint. Reworded
  title. - Miguel ]
Signed-off-by: Miguel Ojeda <[email protected]>

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