Revert most of "fusefrontend: clamp oversized reads"

We cannot return less data than requested to the kernel!

From https://libfuse.github.io/doxygen/structfuse__operations.html:

  Read should return exactly the number of bytes
  requested except on EOF or error, otherwise the
  rest of the data will be substituted with
  zeroes.

Reverts commit 3009ec9852 minus
the formatting improvements we want to keep.

Fixes https://github.com/rfjakob/gocryptfs/issues/147
Reopens https://github.com/rfjakob/gocryptfs/issues/145

internal/fusefrontend/file.go

@@ -132,8 +132,6 @@ func (f *file) createHeader() (fileID []byte, err error) {
 	return h.ID, err
 }
 
-var oversizedReadWarn sync.Once
-
 // doRead - read "length" plaintext bytes from plaintext offset "off" and append
 // to "dst".
 // Arguments "length" and "off" do not have to be block-aligned.
@@ -144,16 +142,6 @@ var oversizedReadWarn sync.Once
 // Called by Read() for normal reading,
 // by Write() and Truncate() for Read-Modify-Write
 func (f *file) doRead(dst []byte, off uint64, length uint64) ([]byte, fuse.Status) {
-	// Our byte cache pools are sized acc. to MAX_KERNEL_WRITE, but the
-	// running kernel may have a higher limit set. Clamp to what we can
-	// handle.
-	if length > fuse.MAX_KERNEL_WRITE {
-		oversizedReadWarn.Do(func() {
-			tlog.Warn.Printf("doRead: truncating oversized read: %d to %d bytes",
-				length, fuse.MAX_KERNEL_WRITE)
-		})
-		length = fuse.MAX_KERNEL_WRITE
-	}
 	// Make sure we have the file ID.
 	f.fileTableEntry.HeaderLock.RLock()
 	if f.fileTableEntry.ID == nil {