configfs lookup race fix

From: Christoph Hellwig
Date: Wed Aug 25 2021 - 02:50:17 EST

Hi all,

this series takes the patch from Sishuai, with the initial refactoring
split into reviewable prep patches and the suggestion from Al taken
into account. It does not fix the pre-existing leak of ->s_dentry and
->d_fsdata that Al noticed yet - that will take a little more time.