proc: Clear the pieces of proc_inode that proc_evict_inode cares about

[ Upstream commit 71448011 ]

This just keeps everything tidier, and allows for using flags like
SLAB_TYPESAFE_BY_RCU where slabs are not always cleared before reuse.
I don't see reuse without reinitializing happening with the proc_inode
but I had a false alarm while reworking flushing of proc dentries and
indoes when a process dies that caused me to tidy this up.

The code is a little easier to follow and reason about this
way so I figured the changes might as well be kept.

Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>
Stable-dep-of: b969f961 ("fix proc_sys_compare() handling of in-lookup dentries")
Signed-off-by: Sasha Levin <sashal@kernel.org>