saner calling conventions for unlazy_child()

[ Upstream commit ae66db45 ]

same as for the previous commit - instead of 0/-ECHILD make
it return true/false, rename to try_to_unlazy_child().

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>