Add SEEK_DATA/SEEK_HOLE constants

Relates to nix-rust/nix#377.  These constants
are only available in systems running the Linux kernel.

Signed-off-by: Paul Osborne <[email protected]>

Author: posborne

src/unix/notbsd/mod.rs

@@ -188,6 +188,8 @@ pub const EOF: ::c_int = -1;
 pub const SEEK_SET: ::c_int = 0;
 pub const SEEK_CUR: ::c_int = 1;
 pub const SEEK_END: ::c_int = 2;
+pub const SEEK_DATA: ::c_int = 3;
+pub const SEEK_HOLE: ::c_int = 4;
 pub const _IOFBF: ::c_int = 0;
 pub const _IONBF: ::c_int = 2;
 pub const _IOLBF: ::c_int = 1;