From: OGAWA Hirofumi <hirofumi@mail.parknet.co.jp>

Current writev() of pipe/fifo can be interleaved with data from other
processes doing writes even when the requests size is <= PIPE_BUF.  These
writes should in fact be atomic.

The readv() side is also supported for same behavior with read().  And it
is faster.

readv/writev version of bw_pipe in LMbench

2.6.0-test9-bk12
hirofumi@devron (i686-pc-linux-gnu)[1010]$ ./bw_pipe -m 4096 -M 5
Pipe bandwidth: 45.53 MB/sec
hirofumi@devron (i686-pc-linux-gnu)[1009]$ ./bw_pipe -m 1024 -M 5
Pipe bandwidth: 20.08 MB/sec

2.6.0-test9-bk12 + patch
hirofumi@devron (i686-pc-linux-gnu)[1001]$ ./bw_pipe -m 4096 -M 5
Pipe bandwidth: 65.98 MB/sec
hirofumi@devron (i686-pc-linux-gnu)[1002]$ ./bw_pipe -m 1024 -M 5
Pipe bandwidth: 32.19 MB/sec