prelloc: warn and continue if fallocate(2) is not supported

This makes gocryptfs work at all on ZFS.
See https://github.com/rfjakob/gocryptfs/issues/22 .

internal/fusefrontend/compat_linux.go

@@ -1,6 +1,13 @@
 package fusefrontend
 
-import "syscall"
+import (
+	"sync"
+	"syscall"
+)
+
+import "github.com/rfjakob/gocryptfs/internal/toggledlog"
+
+var preallocWarn sync.Once
 
 // prealloc - preallocate space without changing the file size. This prevents
 // us from running out of space in the middle of an operation.
@@ -13,6 +20,16 @@ func prealloc(fd int, off int64, len int64) (err error) {
 			// signal and we should try again.
 			continue
 		}
+		if err == syscall.EOPNOTSUPP {
+			// ZFS does not support fallocate which caused gocryptfs to abort
+			// every write operation: https://github.com/rfjakob/gocryptfs/issues/22
+			preallocWarn.Do(func() {
+				toggledlog.Warn.Printf("Warning: The underlying filesystem " +
+					"does not support fallocate(2). gocryptfs will continue working " +
+					"but is no longer resistant against out-of-space errors.\n")
+			})
+			return nil
+		}
 		return err
 	}
 }