pipe - Make pipe r/w MPSAFE, add kern.pipe.mpsafe (disabled by default)

pipe - Make pipe r/w MPSAFE, add kern.pipe.mpsafe (disabled by default)

* Make pipe_read and pipe_write MPSAFE.

* Add a sysctl kern.pipe.mpsafe which defaults to disabled.  Set to
  1 to test the MPSAFE pipe code.  The expectation is that it will be
  set to 1 for the release.  Currently only pipe_read and pipe_write
  is MPSAFE.

* The new code in mpsafe mode also implements a streaming optimization
  to avoid unnecessary tsleep/wakeup/IPIs.  If the reader and writer
  are operating on different cpus this feature results in more uniform
  performance across a wide swath of block sizes.

* The new code currently does not use any page mapping optimizations.
  Page table overhead is fairly nasty on SMP so for now we rely on cpu
  caches and do an extra copy.  This means the code is tuned better for
  more recent cpus and tuned worse for older cpus.  At least for now.

OLD pipe code:  dwe = dwrite_enable, sfb = dwrite_sfbuf mode
NEW pipe code:  mpsafe = 0 (gets bgl) or 1 (does not use bgl)

Using /usr/src/test/sysperf/pipe2.c to test, all results in MBytes/sec

			  8K	 16K	 32K	 64K	128K	256K
			----	----	----	----	----	----
OLD dwe=0		1193	1167	1555	1525	1473	1477
OLD dwe=1 sfb=0		 856    1458	2307	2182	2275	2307
OLD dwe=1 sfb=1		 955	1537	2300	2356	2363	2708
OLD dwe=1 sfb=2		 939	1561	2367	2477	2379	2360

NEW mpsafe=0		1150	1369	1536	1591	1358	1270
NEW mpsafe=1		2133	2319	2375	2387	2396	2418