freezer: unexport refrigerator() and update try_to_freeze() slightly

There is no reason to export two functions for entering the
refrigerator.  Calling refrigerator() instead of try_to_freeze()
doesn't save anything noticeable or removes any race condition.

* Rename refrigerator() to __refrigerator() and make it return bool
  indicating whether it scheduled out for freezing.

* Update try_to_freeze() to return bool and relay the return value of
  __refrigerator() if freezing().

* Convert all refrigerator() users to try_to_freeze().

* Update documentation accordingly.

* While at it, add might_sleep() to try_to_freeze().

Signed-off-by: Tejun Heo <[email protected]>
Cc: Samuel Ortiz <[email protected]>
Cc: Chris Mason <[email protected]>
Cc: "Theodore Ts'o" <[email protected]>
Cc: Steven Whitehouse <[email protected]>
Cc: Andrew Morton <[email protected]>
Cc: Jan Kara <[email protected]>
Cc: KONISHI Ryusuke <[email protected]>
Cc: Christoph Hellwig <[email protected]>

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