coredump: protect corefilename access with a lock

Previously format string traversal could happen while the string itself was
being modified.

Use allproc_lock as coredumping is a rare operation and as such we don't
have to create a dedicated lock.

Submitted by: Tiwei Bie <btw mail.ustc.edu.cn>
Reviewed by: kib
X-Additional: JuniorJobs project