add fallocate (nonstandardized) function

this is equivalent to posix_fallocate except that it has an extra
mode/flags argument to control its behavior, and stores the error in
errno rather than returning an error code.

include/fcntl.h

@@ -116,10 +116,13 @@ struct f_owner_ex {
 	int type;
 	pid_t pid;
 };
+#define FALLOC_FL_KEEP_SIZE 1
+#define FALLOC_FL_PUNCH_HOLE 2
 #define SPLICE_F_MOVE 1
 #define SPLICE_F_NONBLOCK 2
 #define SPLICE_F_MORE 4
 #define SPLICE_F_GIFT 8
+int fallocate(int, int, off_t, off_t);
 ssize_t vmsplice(int, const struct iovec *, size_t, unsigned);
 ssize_t splice(int, off_t *, int, off_t *, size_t, unsigned);
 #define loff_t off_t

src/fcntl/fallocate.c

+#define _GNU_SOURCE
+#include <fcntl.h>
+#include "syscall.h"
+
+int fallocate(int fd, int mode, off_t base, off_t len)
+{
+	return syscall(SYS_fallocate, fd, mode, __SYSCALL_LL_E(base),
+		__SYSCALL_LL_E(len));
+}