Expose SeekData and SeedHole on all Linux targets

These were previously missing on musl and mips targets because of
missing definitions in the libc crate.

src/unistd.rs

@@ -1002,6 +1002,19 @@ pub fn write(fd: RawFd, buf: &[u8]) -> Result<usize> {
     Errno::result(res).map(|r| r as usize)
 }
 
+// Hard-code constants for Linux since they are not exposed on all target by the
+// libc crate. This is fine since these constants are part of the stable kernel
+// ABI.
+cfg_if! {
+    if #[cfg(target_os = "linux")] {
+        const SEEK_DATA: i32 = 3;
+        const SEEK_HOLE: i32 = 4;
+    } else if #[cfg(any(target_os = "dragonfly", target_os = "freebsd"))] {
+        const SEEK_DATA: i32 = libc::SEEK_DATA;
+        const SEEK_HOLE: i32 = libc::SEEK_HOLE;
+    }
+}
+
 /// Directive that tells [`lseek`] and [`lseek64`] what the offset is relative to.
 ///
 /// [`lseek`]: ./fn.lseek.html
@@ -1018,21 +1031,15 @@ pub enum Whence {
     /// Specify an offset relative to the next location in the file greater than or
     /// equal to offset that contains some data. If offset points to
     /// some data, then the file offset is set to offset.
-    #[cfg(any(target_os = "dragonfly", target_os = "freebsd",
-          all(target_os = "linux", not(any(target_env = "musl",
-                                           target_arch = "mips",
-                                           target_arch = "mips64")))))]
-    SeekData = libc::SEEK_DATA,
+    #[cfg(any(target_os = "dragonfly", target_os = "freebsd", target_os = "linux"))]
+    SeekData = SEEK_DATA,
     /// Specify an offset relative to the next hole in the file greater than
     /// or equal to offset. If offset points into the middle of a hole, then
     /// the file offset should be set to offset. If there is no hole past offset,
     /// then the file offset should be adjusted to the end of the file (i.e., there
     /// is an implicit hole at the end of any file).
-    #[cfg(any(target_os = "dragonfly", target_os = "freebsd",
-          all(target_os = "linux", not(any(target_env = "musl",
-                                           target_arch = "mips",
-                                           target_arch = "mips64")))))]
-    SeekHole = libc::SEEK_HOLE
+    #[cfg(any(target_os = "dragonfly", target_os = "freebsd", target_os = "linux"))]
+    SeekHole = SEEK_HOLE,
 }
 
 /// Move the read/write file offset.