Merge fifo_vnops.c:1.118 from HEAD to RELENG_6:

Merge fifo_vnops.c:1.118 from HEAD to RELENG_6:

  Only poll the fifo for read events if the fifo is attached to a readable
  file descriptor.  Otherwise, the read end of a fifo might return that it
  is writable (which it isn't).

  Only poll the fifo for write events if the fifo attached to a writable
  file descriptor.  Otherwise, the write end of a fifo might return that
  it is readable (which it isn't).

  In the event that a file is FREAD|FWRITE (which is allowed by POSIX, but
  has undefined behavior), we poll for both.

Approved by:	re (kensmith)