move fallocate syscall wrapper to linux-specific syscalls dir

src/linux/fallocate.c

@@ -0,0 +1,9 @@
+#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));
+}