locks: clarify posix_locks_deadlock

For such a short function (with such a long comment),
posix_locks_deadlock() seems to cause a lot of confusion.  Attempt to
make it a bit clearer:

	- Remove the initial posix_same_owner() check, which can never
	  pass (since this is only called in the case that block_fl and
	  caller_fl conflict)
	- Use an explicit loop (and a helper function) instead of a goto.
	- Rewrite the comment, attempting a clearer explanation, and
	  removing some uninteresting historical detail.

Signed-off-by: J. Bruce Fields <bfields@citi.umich.edu>