VFS: fix error behavior for partial pipe writes

VFS: fix error behavior for partial pipe writes

This patch fixes two related issues:

- If a large (>PIPE_BUF) pipe write is processed partially, only to be
  followed by a write error condition, then the process is left in an
  incorrect state, possibly causing VFS to crash on a subsequent call.

- If such a partially processed large pipe write ends up resulting in
  an EPIPE error, no corresponding SIGPIPE signal is generated.

The corrected behavior is tested in test68.

Change-Id: I5540e61ab6bcc60a31201485eda04bc49ece2ca8