crash-pipe: Introduce option to copy core using splice(2)

crash-pipe: Introduce option to copy core using splice(2)

Linux splice(2) avoids user-space copy and is effectively faster than
regular read(2) + write(2).

According to (ftrace) test below splice is about 20% faster.

  sysctl -w kernel.core_pattern='|/usr/libexec/core-pipe --save-core /opt/usr/share/temp/core.%p'
  cd /sys/fs/kernel/tracing/
  echo do_coredump > set_ftrace_filter
  echo function_graph > current_tracer
  cat trace_pipe

/* splice */

 2) $ 2183078 us |  } /* do_coredump */
 4) $ 2127946 us |  } /* do_coredump */
 1) $ 2377004 us |  } /* do_coredump */
 1) $ 2088114 us |  } /* do_coredump */

/* read/write */
 1) $ 3088297 us |  } /* do_coredump */
 1) $ 2953330 us |  } /* do_coredump */
 1) $ 2647784 us |  } /* do_coredump */
 3) $ 2271315 us |  }

Change-Id: Id2c681a364ce96d98c70329abdf4a64f6d0b405d