PhysicalMemory: open /tmp lock file with O_NOFOLLOW (fix symlink redirect)

Summary:
`PhysicalMemory`'s constructor opened its lock file `/tmp/pmem_<phyAddr>_lock` with `O_CREAT | O_WRONLY` (no `O_NOFOLLOW`). The path is predictable from the device tree, and this process runs as root, so on an image where an unprivileged user can write to `/tmp` a local attacker could pre-place the path as a symlink to a sensitive root-writable target and have the `O_CREAT | O_WRONLY` open follow the link.

This adds `O_NOFOLLOW` (the open refuses to follow a pre-planted symlink) and `O_CLOEXEC` (avoid leaking the lock fd across exec). The file remains a shared advisory lock reused across processes, so `O_EXCL` is intentionally not used.

Addresses a LOW finding from the FBOSS AI security scan (AVDP).

Reviewed By: zechengh09

Differential Revision: D114768813

fbshipit-source-id: 7916bd3e7e3aee79bfad5fb394f3dfb0a7ec22c1

Author: srikrishnagopu

fboss/lib/PhysicalMemory.cpp

@@ -43,8 +43,10 @@ PhysicalMemory::PhysicalMemory(uint64_t phyAddr, uint32_t size, bool mustLock)
   // construct the lock file name
   const auto lockFN = fmt::format("{}/pmem_{:x}_lock", kLockPath, phyAddr_);
 
-  // lock it
-  lockFile_ = folly::File(lockFN, O_CREAT | O_WRONLY);
+  // lock it. O_NOFOLLOW refuses a pre-planted symlink at the predictable /tmp
+  // lock path (this process runs as root), and O_CLOEXEC avoids leaking the fd
+  // across exec.
+  lockFile_ = folly::File(lockFN, O_CREAT | O_WRONLY | O_NOFOLLOW | O_CLOEXEC);
   auto locked = lockFile_.try_lock();
   XLOG(DBG1) << folly::format(
       "{} to acquire lock, {}({})",