Revert "proc: allow pid_revalidate() during LOOKUP_RCU"

This reverts commit da4d6b9c.

Appears to cause stale dcache entries for inodes in /proc/$pid
filesystem resulting in ESRCH failures on access to files
under /proc/self

Bug: 305682813
Bug: 306055483
Signed-off-by: Todd Kjos <tkjos@google.com>
Change-Id: I004338c043d7e777fdfef343e45ccb20ff4d503b