Main part here is making parallel lookups safe for RT - making

sure preemption is disabled in start_dir_add()/ end_dir_add() sections (on
non-RT it's automatic, on RT it needs to to be done explicitly) and moving
wakeups from __d_lookup_done() inside of such to the end of those sections.
	Wakeups can be safely delayed for as long as ->d_lock on in-lookup
dentry is held; proving that has caught a bug in d_add_ci() that allows
memory corruption when sufficiently bogus ntfs (or case-insensitive xfs)
image is mounted.  Easily fixed, fortunately.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
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