swait: clarify swait should mostly be avoided

swait documentation had an open invitation suggesting that it should be
used where possible. Linus and Kristen have pointed out a slew of reasons
[0] [1] why it should be the other way around, swait should be avoided at
all costs and swait then should only be used in very special circumstances.

Since swait should only be used in very special circumstances we prevent
modular use. Later if a selftest / debug module creeps up we could just:

  #if !defined(MODULE) || defined(__DEBUG_ENABLE_SWAIT)

[0] https://lkml.kernel.org/r/CA+55aFykNULx-b6M6FmUYdK2cn-OJKKfjaPwLN5xZGK+bioGaA@mail.gmail.com
[1] https://lkml.kernel.org/r/20170630173048.GA2392@templeofstupid.com

Cc: Matthew Wilcox <mawilcox@microsoft.com>
Cc: Krister Johansen <kjlx@templeofstupid.com>
Cc: Marcelo Tosatti <mtosatti@redhat.com>
Cc: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
Cc: Marcelo Tosatti <mtosatti@redhat.com>
Suggested-by: Linus Torvalds <torvalds@linux-foundation.org>
Suggested-by: Matthew Wilcox <mawilcox@microsoft.com>
Signed-off-by: 	Davidlohr Bueso <dave@stgolabs.net>
Signed-off-by: Luis R. Rodriguez <mcgrof@kernel.org>