macos: fix O_DIRECT build failure

O_DIRECT has no direct equivalent on MacOS
(check out https://github.com/libuv/libuv/issues/1600 for details).

Just define it to zero there.

internal/fusefrontend/fs.go

@@ -105,7 +105,7 @@ func (fs *FS) mangleOpenFlags(flags uint32) (newFlags int) {
 	// crypto header, alignment will be off, even if userspace makes aligned
 	// accesses. Running xfstests generic/013 on ext4 used to trigger lots of
 	// EINVAL errors due to missing alignment. Just fall back to buffered IO.
-	newFlags = newFlags &^ syscall.O_DIRECT
+	newFlags = newFlags &^ syscallcompat.O_DIRECT
 
 	return newFlags
 }

internal/syscallcompat/sys_darwin.go

@@ -9,6 +9,10 @@ import (
 	"github.com/hanwen/go-fuse/fuse"
 )
 
+// O_DIRECT means oncached I/O on Linux. No direct equivalent on MacOS and defined
+// to zero there.
+const O_DIRECT = 0
+
 // Sorry, fallocate is not available on OSX at all and
 // fcntl F_PREALLOCATE is not accessible from Go.
 // See https://github.com/rfjakob/gocryptfs/issues/18 if you want to help.

internal/syscallcompat/sys_linux.go

@@ -14,6 +14,10 @@ import (
 
 const _FALLOC_FL_KEEP_SIZE = 0x01
 
+// O_DIRECT means oncached I/O on Linux. No direct equivalent on MacOS and defined
+// to zero there.
+const O_DIRECT = syscall.O_DIRECT
+
 var preallocWarn sync.Once
 
 // EnospcPrealloc preallocates ciphertext space without changing the file