t6913679: Use fallocate on LOG FILESS

Summary: Use fallocate on LOG FILES to

Test Plan:
make check
+
===check with strace===

[arya@devvm1441 ~/rocksdb] strace -e trace=fallocate ./ldb --db=/tmp/test_new scan
fallocate(3, 01, 0, 4194304)            = 0

Reviewers: sdong, anthony, IslamAbdelRahman, kradhakrishnan, lgalanis, rven, igor

Reviewed By: igor

Subscribers: leveldb, dhruba

Differential Revision: https://reviews.facebook.net/D45969

util/env_posix.cc

@@ -1134,6 +1134,9 @@ class PosixEnv : public Env {
       return IOError(fname, errno);
     } else {
       int fd = fileno(f);
+#ifdef ROCKSDB_FALLOCATE_PRESENT
+      fallocate(fd, FALLOC_FL_KEEP_SIZE, 0, 4 * 1024 * 1024);
+#endif
       SetFD_CLOEXEC(fd, nullptr);
       result->reset(new PosixLogger(f, &PosixEnv::gettid, this));
       return Status::OK();