file-posix: Do runtime check for ofd lock API

It is reported that on Windows Subsystem for Linux, ofd operations fail
with -EINVAL. In other words, QEMU binary built with system headers that
exports F_OFD_SETLK doesn't necessarily run in an environment that
actually supports it:

$ qemu-system-aarch64 ... -drive file=test.vhdx,if=none,id=hd0 \
    -device virtio-blk-pci,drive=hd0
qemu-system-aarch64: -drive file=test.vhdx,if=none,id=hd0: Failed to unlock byte 100
qemu-system-aarch64: -drive file=test.vhdx,if=none,id=hd0: Failed to unlock byte 100
qemu-system-aarch64: -drive file=test.vhdx,if=none,id=hd0: Failed to lock byte 100

As a matter of fact this is not WSL specific. It can happen when running
a QEMU compiled against a newer glibc on an older kernel, such as in
a containerized environment.

Let's do a runtime check to cope with that.

Reported-by: Andrew Baumann <[email protected]>
Reviewed-by: Eric Blake <[email protected]>
Signed-off-by: Fam Zheng <[email protected]>
Signed-off-by: Kevin Wolf <[email protected]>

block/file-posix.c

@@ -457,22 +457,19 @@ static int raw_open_common(BlockDriverState *bs, QDict *options,
     switch (locking) {
     case ON_OFF_AUTO_ON:
         s->use_lock = true;
-#ifndef F_OFD_SETLK
-        fprintf(stderr,
-                "File lock requested but OFD locking syscall is unavailable, "
-                "falling back to POSIX file locks.\n"
-                "Due to the implementation, locks can be lost unexpectedly.\n");
-#endif
+        if (!qemu_has_ofd_lock()) {
+            fprintf(stderr,
+                    "File lock requested but OFD locking syscall is "
+                    "unavailable, falling back to POSIX file locks.\n"
+                    "Due to the implementation, locks can be lost "
+                    "unexpectedly.\n");
+        }
         break;
     case ON_OFF_AUTO_OFF:
         s->use_lock = false;
         break;
     case ON_OFF_AUTO_AUTO:
-#ifdef F_OFD_SETLK
-        s->use_lock = true;
-#else
-        s->use_lock = false;
-#endif
+        s->use_lock = qemu_has_ofd_lock();
         break;
     default:
         abort();