[PATCHES v2][CFT] rename deadlock fixes

[Al Viro]

Updated variant forced-pushed into #work.rename; changes since v1:
	* rebased on top of #merged-selinux, to avoid a bisection hazard;
selinuxfs used to abuse lock_rename() badly enough to trigger the checks
in the last commit.  Fixed in #merged-selinux (which consists of a commit
already in selinux git tree), and rebase on top of that does not require
any changes in the series.
	* (hopefully) fixed the markup in directory-locking.rst.  I would
really appreciate if somebody familiar with reST took a look at that.
	* fixed another part of directory-locking.rst, not touched in v1 -
the proof that operations will not introduce loops if none had been present
should've been updated when RENAME_EXCHANGE got introduced; it hadn't been.
Rewritten.

Same branch, same overall description, individual patches in followups.