Implement posix_fallocate

`posix_fallocate` ensures that a byte range in a file is allocated so
that subsequent writes don't fail. Unlike ftruncate, posix_fallocate
does not shrink files.

The Linux syscall fallocate is similar to posix_fallocate except with
far more control over how byte ranges are allocated (e.g. it supports
file holes and other features). This MR doesn't implement fallocate as
it requires syscall and redoxfs support.

Finally, I changed the flags for flock from usize to c_int. That matches
what we have in libc and also avoids some silly, needless type casting.

src/platform/linux/mod.rs

@@ -1,4 +1,4 @@
-use core::{arch::asm, ptr};
+use core::{arch::asm, num::NonZeroU64, ptr};
 
 use super::{ERRNO, Pal, types::*};
 use crate::{
@@ -560,6 +560,11 @@ impl Pal for Sys {
         e_raw(unsafe { syscall!(PIPE2, fildes.as_mut_ptr(), flags) }).map(|_| ())
     }
 
+    fn posix_fallocate(fd: c_int, offset: u64, length: NonZeroU64) -> Result<()> {
+        let length = length.get();
+        e_raw(unsafe { syscall!(FALLOCATE, fd, 0, offset, length) }).map(|_| ())
+    }
+
     fn posix_getdents(fildes: c_int, buf: &mut [u8]) -> Result<usize> {
         let current_offset = Self::lseek(fildes, 0, SEEK_CUR)? as u64;
         let bytes_read = Self::getdents(fildes, buf, current_offset)?;